For your perusal, the output of dmesg on the EAP670, showing what I think are the failure events (at the bottom).

I was going to just include the text here, but your system says " Content or title contains illegal external links."

https://drive.google.com/file/d/1TecPhP9jnj5DXLJ9wWXqfMMVpFs8rVcs/view?usp=sharing

Extract:

[  111.340224] wlan: [0:I:ANY] ol_ath_vap_set_param: Now supported BCAST RATE is 6000(kbps) rate code: 0x10000003
[  111.345449] wlan: [0:I:ANY] ol_ath_vap_set_param: Now supported MCAST RATE 6000(kbps), rate code: 0x10000003
[  118.810242] wlan: [624:I:ANY] wlan_cfg80211_set_ap_chanwidth: wlan_cfg80211_set_ap_chanwidth: freq:5745 hw_value:149 ch_width:3
[  181.616424] wlan: [351:I:ANY]  ieee80211_ioctl_kickmac[8421]
[  181.616424]
[  302.016207] wlan: [351:I:ANY]  ieee80211_ioctl_kickmac[8421]
[  302.016207]
[ 2068.994359] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[ 2961.603142] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[12865.196164] wlan: [351:I:ANY]  ieee80211_ioctl_kickmac[8421]
[12865.196164]
[18445.732549] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[23684.841701] wlan: [0:E:ANY] ol_ath_send_peer_assoc: Node 48:a2:e6:04:8a:fa does not support HT but htcaps are populated
[23684.841701]
[23684.841753] wlan: [0:E:ANY] ol_ath_send_peer_assoc: Node Flags: 0x0 , ht_caps: 0x110c
[23684.841753]
[25172.598261] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[25700.258530] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[26481.708038] wlan: [0:E:NSS] [nss-wifili]: NSS peer auth update message failed with error = 95 peer_id 65535 auth 0
[29168.414922] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[32837.573888] wlan: [0:E:NSS] [nss-wifili]: NSS peer auth update message failed with error = 95 peer_id 65535 auth 0
[33346.078137] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[45133.488506] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[51707.288002] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[52324.152108] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[52484.173941] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[56943.986616] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[57036.611002] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[57292.070065] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[57324.998976] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context
[57412.350443] wlan: [0:I:ANY] ieee80211_complete_wbuf: ieee80211_complete_wbuf: Reo queue update failed
[59361.439207] wlan: [0:E:NSS] [nss-wifili]: peer NULL - Null nss context

  @MR.S I've wasted entirely too much time on this.  I have tried simplifying the topology, changing cables, using the power adapter, etc. and still have stability issues.

What bothers me most is the lack of meaningful logging and diagnostic capability.

I will return the device to Amazon for a refund.

  @jhg6308 Been having the same exact issue over the past few months. It's been driving me crazy, and as you said, no diagnostics can be generated.

I've downgraded firmware on the AP, powered via DC, changed edge switches, swapped patch cables; you name it I've done it.
I have an EAP773 that's rock solid, but I've had constant issues with this EAP670 v1.26 - too bad I'm out of my return window. I'll just have to sell on eBay for a loss I suppose.

What's extremely frustrating is when devices are "connected" but with no throughput.

Thank you for your efforts and documentation to ease my frustration with this access point! I really appreciate your time.

No sense in wasting my time with TP-Link support, I know it'll just be a run-around with a potential exchange for the same model with the same problem.

  @FrostTech I was still within the 30-day Amazon return window, so I gave up on TP-Link and went with Ubiquiti U7-Pro, which is working great.  A bit more money but the management console software is MUCH more professional.  Also ssh login on the device is full root access and there are good diagnostic and troubleshooting tools. IOW, TP-Link treats the user as a dumb consumer while Ubiquiti treats you as a professional.