Do you know if its firmware is fully up to date or not?
What does it say on it's Administration-> System page?
I doubt old firmware would prevent operation.
When you set up the Asus did you use the wizard or do it manually?
Hoping you would be along @TimCurtis
UBR without PCR
UBR with PCR
CBR
Vbr
Gfr
NRT
I have set this as per the old modem
Manually setup
I happened to notice the old modem/router didnt seem to be using IPv6, what leads me to be looking at this configuration page.
maybe something here isnt correct?
Use the wizard selecting the type of service you have and then add in the extras you need like username & password and your wireless settings.
Factory reset first to start with a clean slate.
Before doing any of those, Make sure it has the latest firmware, yes even Asus themselves say to update the firmware first. They know their own equipment better than us:
I will be shot down for saying this, but I'm going to post it anyway:
DSL-AX82U|Modem Routers|ASUS United Kingdom
Even if the firmware is not intended to fix possible DSL connection issues, there are updates to make IPV6 work more smoothly in there. Routers are one of the important things you always want up to date, for security if not anything else. Strange that this was not suggested here.
So you may as well update before you reset it 😊