Simple test then.
Power off your RP-AX58s, reboot your RT-AX92Us, re-scan the wireless networks on your phone and tablet.
Then note there's no Wi-Fi 6 symbol on the 2.4GHz and 5GHz-1 SSIDs.
Case closed. Please thank your mother for the rabbits.
I wonder if it's the radios of the RP-AX58s, being 11ax on both bands, that are the reason the 'adelphiaUK.Smart' and 'adelphiaUK.Smart5G.1' show as 'Wi-Fi6'?
Could be bollocks reporting by the phone. Only one of the three radios is 11ax/Wi-F 6 capable.
Use WiFiMan wireless analyser app (or similar) on your phone and scan your network. Pick your 2.4GHz SSID, then select the access point. What does 'PHY Mode' for the access point show as?
Similarly...
Sorry, I misunderstood.
The RT-AX92U spec here for us Aussies has the 11ax/5GHz-2 radio of the RT-AX92U, operating on the higher-powered channels 149-165 (whereas the 11ac/5GHz-1 radio operates on the lower channels 36-64).
I think you're in pommie-land, in which case I don't believe you get...
Have you tried 'continuous' DHCP Query Frequency?
Edit -
Yes you have:
'With newer versions (e.g., 3.0.0.4.388_24403), tried normal, aggressive, and continuous modes — none of them work'
From an old thread on this same subject, but I've no idea if changing the territory_code will work and persist (post #20) -
https://www.snbforums.com/threads/can-i-remove-oem-plugin-and-ensure-a-clean-system.70206/post-912572
Should be easy to identify the product region, in a telnet/SSH session on the router run -
nvram get territory_code
If the resultant value is 'CN' then I'm sure it was intended for the Chinese market.
Why do you want to try that instead of simply installing GNUton's DSL-AX82U firmware?
(but 388.7_1-gnuton1 not 388.8_2-gnuton1 apparently as per - https://github.com/gnuton/asuswrt-merlin.ng/issues/700)
The laptop and printer could be on different subnets.
Why do you need to use the the ISP router? Alternatively, if you need to keep using the the ISP router then put the "main" ASUS (XT8?) in its "AiMesh Router in AP mode", with its IP address in the same subnet as the ISP router.