5GHZ band randomly resets and drops connection for anything connected to it. The 2.4GHZ band can be switched to and will still work. This happens at least once a day, if not more, at seemingly random times. There is no issue with the power adapter (it's the original), and I tried it on its own outlet, but that did not help, in case it were an issue.

(NOTE: This also happened to a v1 version of the same router for me that I could only make work by going to a previous firmware. I do not want to do that since the latest for v3 had a security fix.)

Is there a later beta after 2018 for this router that fixes this issue?

This results in this set of errors in the log (note the weird dates of 1970 and 2017):

Hardware Version: ArcherC9 v3.0
Software Version: 1.3.1 Build 20171215 rel.35219

2021-07-06 11:52:40 locale[14901]: <6> 283051 Language is changed to 'en_US'
2021-07-06 11:52:40 locale[14901]: <7> 283002 Explorer language is 'en_US'
2021-07-06 11:38:41 led-controller[912]: <6> 288051 Start to run WAN1_ON
2021-07-06 11:38:41 led-controller[912]: <7> 288004 WAN1_ON is found
2021-07-06 11:38:41 led-controller[912]: <7> 288003 Led rule name is WAN1_ON
2021-07-06 11:38:41 led-controller[912]: <7> 288001 Receive data length = 132
2021-07-06 11:38:41 led-controller[912]: <6> 288051 Start to run WAN0_OFF
2021-07-06 11:38:41 led-controller[912]: <7> 288004 WAN0_OFF is found
2021-07-06 11:38:41 led-controller[912]: <7> 288003 Led rule name is WAN0_OFF
2021-07-06 11:38:41 led-controller[912]: <7> 288001 Receive data length = 132
2021-07-06 11:38:41 led-controller[912]: <6> 288051 Start to run LAN_ON
2021-07-06 11:38:41 led-controller[912]: <7> 288004 LAN_ON is found
2021-07-06 11:38:41 led-controller[912]: <7> 288003 Led rule name is LAN_ON
2021-07-06 11:38:41 led-controller[912]: <7> 288001 Receive data length = 132
2021-07-06 11:38:36 locale[8077]: <6> 283051 Language is changed to 'en_US'
2021-07-06 11:38:36 locale[8077]: <7> 283002 Explorer language is 'en_US'
2021-07-06 11:38:04 locale[7714]: <6> 283051 Language is changed to 'en_US'
2021-07-06 11:38:04 locale[7714]: <7> 283002 Explorer language is 'en_US'
2021-07-06 11:38:03 locale[7653]: <6> 283051 Language is changed to 'en_US'
2021-07-06 11:38:03 locale[7653]: <7> 283002 Explorer language is 'en_US'
2021-07-06 11:37:57 led-controller[912]: <6> 288051 Start to run WAN1_ON
2021-07-06 11:37:57 led-controller[912]: <7> 288004 WAN1_ON is found
2021-07-06 11:37:57 led-controller[912]: <7> 288003 Led rule name is WAN1_ON
2021-07-06 11:37:57 led-controller[912]: <7> 288001 Receive data length = 132
2021-07-06 11:37:57 led-controller[912]: <6> 288051 Start to run WAN0_OFF
2021-07-06 11:37:57 led-controller[912]: <7> 288004 WAN0_OFF is found
2021-07-06 11:37:57 led-controller[912]: <7> 288003 Led rule name is WAN0_OFF
2021-07-06 11:37:57 led-controller[912]: <7> 288001 Receive data length = 132
2021-07-06 11:37:57 led-controller[912]: <6> 288051 Start to run LAN_ON
2021-07-06 11:37:57 led-controller[912]: <7> 288004 LAN_ON is found
2021-07-06 11:37:57 led-controller[912]: <7> 288003 Led rule name is LAN_ON
2021-07-06 11:37:57 led-controller[912]: <7> 288001 Receive data length = 132
2017-12-15 00:00:33 led-controller[912]: <6> 288051 Start to run WAN1_OFF
2017-12-15 00:00:33 led-controller[912]: <7> 288004 WAN1_OFF is found
2017-12-15 00:00:33 led-controller[912]: <7> 288003 Led rule name is WAN1_OFF
2017-12-15 00:00:33 led-controller[912]: <7> 288001 Receive data length = 132
2017-12-15 00:00:33 led-controller[912]: <6> 288051 Start to run WAN0_ON
2017-12-15 00:00:33 led-controller[912]: <7> 288004 WAN0_ON is found
2017-12-15 00:00:33 led-controller[912]: <7> 288003 Led rule name is WAN0_ON
2017-12-15 00:00:33 led-controller[912]: <7> 288001 Receive data length = 132
2017-12-15 00:00:33 led-controller[912]: <6> 288051 Start to run LAN_ON
2017-12-15 00:00:33 led-controller[912]: <7> 288004 LAN_ON is found
2017-12-15 00:00:33 led-controller[912]: <7> 288003 Led rule name is LAN_ON
2017-12-15 00:00:33 led-controller[912]: <7> 288001 Receive data length = 132
2017-12-15 00:00:29 qos[7015]: <6> 259504 Service start
2017-12-15 00:00:29 qos[7015]: <6> 259503 Function disabled
2017-12-15 00:00:19 factory-reset[5537]: <6> 284504 Service start
2017-12-15 00:00:19 parental-control[5467]: <6> 229504 Service start
2017-12-15 00:00:19 parental-control[5467]: <6> 229503 Function disabled
2017-12-15 00:00:18 access-control[5408]: <6> 239504 Service start
2017-12-15 00:00:18 access-control[5408]: <6> 239503 Function disabled
2017-12-15 00:00:17 nat[4871]: <7> 211001 Flush conntrack
2017-12-15 00:00:17 nat[4871]: <6> 211502 Function enabled
2017-12-15 00:00:17 nat[4871]: <5> 211051 Create NAT chain succeeded
2017-12-15 00:00:16 nat[4871]: <6> 211501 Initialization succeeded
2017-12-15 00:00:16 nat[4871]: <6> 211501 Initialization succeeded
2017-12-15 00:00:15 basic-security[4375]: <6> 219504 Service start
2017-12-15 00:00:15 basic-security[4375]: <5> 219606 Flush conntrack table succeeded
2017-12-15 00:00:14 firewall[1927]: <6> 209504 Service start
2017-12-15 00:00:08 imb[2782]: <6> 218507 Daemon connection succeeded
2017-12-15 00:00:08 imb[2782]: <6> 218012 ARP Binding disabled
2017-12-15 00:00:08 imb[2782]: <6> 218506 Config interface initialization succeeded
2017-12-15 00:00:08 imb[2782]: <6> 218501 Initialization succeeded
2017-12-15 00:00:07 upnp[2597]: <6> 217504 Service start
2017-12-15 00:00:07 upnp[2597]: <6> 217505 Service stop
2017-12-15 00:00:00 led-controller[916]: <6> 288051 Start to run STATUS_SAN
2017-12-15 00:00:00 led-controller[916]: <7> 288004 STATUS_SAN is found
2017-12-15 00:00:00 led-controller[916]: <7> 288003 Led rule name is STATUS_SAN
2017-12-15 00:00:00 led-controller[916]: <7> 288001 Receive data length = 132
1970-01-01 00:00:10 led-controller[916]: <6> 288504 Service start
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led WLAN5G
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led WLAN2G
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led STATUS
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led WAN2
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led WAN1
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led WPS
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led USB3
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led USB1
1970-01-01 00:00:10 led-controller[916]: <7> 288002 Adding led LAN

Thanks!

Can anyone like @Kevin_Z shed any light?