You do have that troubled SFT1200
The SIFlower hardware SF16a29 and driver are unique and closed source. It seems like they cannot make it work. Either the signal is strong and unstable, or it is stable but very very weak.
The variations are even in the different 4.7.2beta releases. (beta1,beta2 and beta3)
4.8.3 beta was stable but very weak. They removed it again!
At least the 4.8.3 had more options to reduce the connection tracking disasterous implementation.
Because the GL.Inet implementation makes it all worse.
If the connection has a glitch (you lost 20 pings as seen in you log) then the reset of the connection is a cure worse than the disease.
It drops the firewall and all IP connections, in case of a short interruption (interference?) or even when volontary roaming is done.
Stop using connection tracking, it makes no sense if there is only one possible uplink, to stop MWAN3 track disconnects
Give only one SSID for the uplink, and set the BSSID as locked, to stop roaming.(which brings yet another firewall reset and major disconnect)
For 5GHz (more stability and less interference) have no SSID for AP's that operate in the DFS channel range in the list (SFT1200 will not connect to DFS with most 4.x firmwares)
And we hope for the next 4.8.3beta with better power, as the extra options in this beta are great for uplink stability.