I have a GP-AP1300LTE that has been having an issue ever since V3.203 was installed.
I have a handful of IOT devices (smart WiFi switches, etc) that use an ESP8266 chip. These devices only support 2Ghz wifi, and they are unable to connect to the router with V3.203 installed.
However, if I install V3.105, the devices can again connect.
Sorry that I forgot to check up on this post. I just stuck with the older firmware until I saw that the Flint was available and decided to upgrade to WiFi6. The Flint also has this same problem, but I am using an AR150 in AP mode connected to a port bridged to Guest as a workaround.