Could be other ecobee 5G devices as well. Symptoms were that it would connect to the AP but would fail on DHCP (view in ecobee Settings > Wi-Fi > Wi-Fi Logs) and be stuck at “connecting to ecobee<dot>com…”. It would pull a 164. IP on DHCP fail.
I upgraded an AP and have been banging my head against the wall trying to get the ecobee to connect to the network again. I found a reddit post that pointed me in the right direction (Google: PSA: ecobee 5 (Smart Thermostat with Voice Control) does not support 5GHz via DFS channels).
Even after disabling the 5G network and only broadcasting 2.4G, it would still not connect, even after pulling it from the wall several times. I had to go into Settings > Wi-Fi > Wi-Fi Radio > Disable, and then re-enable the radio before it would re-connect to the 2.4G SSID.
From reddit post:
Further to my earlier post, we discovered that the newest ecobee—which is the only model that supports connecting to wifi over the 5GHz band—will not connect to DFS channels.
This means that you need to go into your router/access point setup and make sure that your 5GHz band is on a non-DFS channel, or the ecobee won’t be able to get an IP via DHCP. For me, DFS channels are 52-144.
Fun fact:Dynamic Frequency Selection (DFS) is a WiFi function that enables WLANs to use 5 GHz frequencies that are generally reserved for radars. One main benefit of using DFS channels is to utilize under-serviced frequencies to increase the number of available WiFi channels.