Thanks for the warm welcome.
I think I'm still running 384 firmware as I recently hard reset my router due to the issues, and forgot to upgrade it again. I'll give the latest merlin firmware a shot and test further. Thanks!
I'm currently using my RT-AC5300 in repeater mode, where it randomly disconnects from the parent AP (tplink AX73) due to a "wrong password"(but password was never changed).I can still connect to the wifi, but I can't access the internet. To fix this, I have to rescan AP or switch my router off...