R8000 - connectivity issues

Hi Team

X6 R8000 router

4MKC997AA02AC

Firmware V1.0.4.68_10.1.75

Internet speed test "Good", phone app shows111.33 down, 9.8 up

Panel lights:

Power: Solid white

Internet: flashing white

2.4: solid white/flashing white

5:solid white/flashing white

And ethernet connections all white

Problem:

I'm using the "Smart connect" function (and have tried with it off) and our network devices are all experiencing bad connectivity and numerous drop outs. The "home testing" on the phone app shows huge variance with connection rates, 95% signal and only 6Mbps, press again and 95% signal and up to 108Mbps from the same location (say lounge).

If however I enable a guest network and allow open access (no PW) all devices have no issue connecting to this and retain a stable connection, but at only 2.4 which is expected - even in the more remote areas of the coverage.

I've scanned channels and tried both the "any" option and also fixed the channel to the "great" one. No difference.

Any ideas?

Rob