I don’t know if it’s the same with Sky Q, but on the regular Sky service you’re not allowed to replace the router. They don’t release the DSL credentials, the username and password is hard-coded into the router.
There’s ways around this, or at least used to be. There used to be a website where you could enter the serial number of the router and it’ll spit out the credentials, but whether that still exists / works with modern kit I don’t know. I could Google but, well, so can you. (-:
If you can get the username and password, there’s nothing to physically stop you from replacing it. Just keep the old one to hand in case you ever have to call Support. If you can’t get the creds, you’re humped I’m afraid.