When running the tests, it usually starts at higher speeds (350-380 mbps DL) and then drops rapidly by 10's to 250 or so usually. Via wired connection to the PC, the speeds are stable at 380-390 mbps...

Yep, as mentioned in initial post, all of the drivers are up to date alongside router itself.

Hey guys, I have an issue with my new AX53 router. So, prior to upgrading I did not have Wifi 6 on router and I was looking forward to getting it, to give you some perspective, I pay for 400 mbps up...