Hello @Joseph-TP
Yes, plan is 100Mbps. I use the Archer GE800 for 10Gb LAN connection to the NAS, parental controls, and in the future for other functions, but that is not necessary.
Connecting to 5G wifi is not important for me now. I mainly use a wired LAN connection between PC - Archer - NAS.
The computer was turned off all day today and right after it was turned on (after 18:00) I turned on speedtest.net on it and checked the speed. The speed was 42/15. Then I disconnected and reconnected the WAN cable and measured the speed again. The speed reached 88/88. I am attaching print screens.
Connection from my provider is OK (tested another router Archer C20)
To tips:
Last week I tried several times with QoS turned off and on, but it had no effect on the WAN speed drop. It still appears randomly.
I need to have parental control turned on.
WIFI connection is not important, I use wired LAN. When the speed drop on WAN is also reflected on WIFI.
I will test UPNP turned off. I did not find the second mentioned SIP ALG function in the settings.
After loading Windows:

After unplug-plug WAN cable:

Today system protocol log:
2025-09-22 18:15:19 Ovladač LED INFORMACE [22120] Start to run INET_UP
2025-09-22 18:15:05 QoS INFORMACE [25192] Service start
2025-09-22 18:15:04 UPnP INFORMACE [26428] Service start
2025-09-22 18:15:04 UPnP INFORMACE [26428] Service stop
2025-09-22 18:15:04 Vzdálená správa INFORMACE [26108] Service stop
2025-09-22 18:15:04 NAT UPOZORNĚNÍ [24307] The address xxx.xxx.x.xxx will be DMZ host
2025-09-22 18:15:04 NAT INFORMACE [24307] DMZ enabled
2025-09-22 18:15:03 QoS INFORMACE [25192] Function enabled
2025-09-22 18:15:03 QoS INFORMACE [25192] Service stop
2025-09-22 18:15:02 NAT INFORMACE [24307] Initialization succeeded
2025-09-22 18:15:02 NAT INFORMACE [24307] Initialization succeeded
2025-09-22 18:15:00 Ovladač LED INFORMACE [22120] Start to run INET_OFF
2025-09-22 18:15:00 DHCPC INFORMACE [23995] receive ack from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=12496;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)
2025-09-22 18:15:00 DHCPC INFORMACE [23995] send select request with options(cliid=xx/xx:xx:xx:xx:xx:xx:;reqip=xxx.xxx.xx.xx;serverid=xxx.xx.x.xxx;)
2025-09-22 18:15:00 DHCPC INFORMACE [23995] receive offer from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=12496;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)
2025-09-22 18:15:00 DHCPC INFORMACE [23995] send discover with ip 0.0.0.0 and flags 0
2025-09-22 18:14:58 Ovladač LED INFORMACE [22120] Start to run INET_OFF
2025-09-22 18:14:57 NAT INFORMACE [23009] Initialization succeeded
2025-09-22 18:14:57 NAT INFORMACE [23009] Initialization succeeded
2025-09-22 18:14:56 DHCPC VAROVÁNÍ [22232] send dhcp release ip xxx.xxx.xx.xx
2025-09-22 18:14:56 DHCPC INFORMACE [22232] teardown and release
2025-09-22 18:06:29 Ovladač LED INFORMACE [22120] Start to run wanlancombophy_ON wanlancombosfp_ON lan2g5_1_ON lan2g5_2_OFF lan2g5_3_OFF
2025-09-22 17:43:16 DHCPC INFORMACE [22232] receive ack from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=14400;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)
2025-09-22 17:43:16 DHCPC INFORMACE [22232] send select request with options(cliid=xx/xx:xx:xx:xx:xx:xx:;)
2025-09-22 17:43:16 DHCPC INFORMACE [22232] 1/2 lease passed, enter renewing state
2025-09-22 15:43:12 DHCPC INFORMACE [22232] receive ack from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=14400;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)
2025-09-22 15:43:12 DHCPC INFORMACE [22232] send select request with options(cliid=xx/xx:xx:xx:xx:xx:xx:;)
2025-09-22 15:43:12 DHCPC INFORMACE [22232] 1/2 lease passed, enter renewing state
2025-09-22 13:43:09 DHCPC INFORMACE [22232] receive ack from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=14400;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)
2025-09-22 13:43:09 DHCPC INFORMACE [22232] send select request with options(cliid=xx/xx:xx:xx:xx:xx:xx:;)
2025-09-22 13:43:09 DHCPC INFORMACE [22232] 1/2 lease passed, enter renewing state
2025-09-22 11:43:06 DHCPC INFORMACE [22232] receive ack from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=14400;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)
2025-09-22 11:43:06 DHCPC INFORMACE [22232] send select request with options(cliid=xx/xx:xx:xx:xx:xx:xx:;)
2025-09-22 11:43:06 DHCPC INFORMACE [22232] 1/2 lease passed, enter renewing state
2025-09-22 09:43:02 DHCPC INFORMACE [22232] receive ack from server with ip xxx.xxx.xx.xx, options(serverid=xxx.xx.x.xxx;lease=14400;subnet=255.255.255.0;router=xxx.xxx.xx.x;dns=xx.xxx.xx.xxx xx.xxx.xx.xxx;)