I'm now with NowTV, though only ADSL. Switched due to me getting nowhere with trying to get a fault fixed with my previous provider, and I can now say that Now TV's technical support seems pretty good in comparison.
As you are perhaps aware, all NowTV broadband services are simply rebranded Sky services, sold at different prices and with different contract terms but with all technical aspects of the connection identical to that of a Sky subscriber.
In common with all mass-market providers, if you need technical support they will ask you to use their provided router. Using your own router is otherwise perfectly possible. The only caveat with Sky/NowTV VDSL connections is that the connection isn't PPPoE like most other providers in the UK, it is straight up IPoE (internet protocol over ethernet) authenticated using DHCP Option 61. Not all routers support DHCP Option 61, but I believe the Asus Merlin firmware does (you may wish to check). Once upon a time you had to capture the authentication credentials from their supplied router since they wouldn't tell you them, but nowadays you can just use generic credentials. For what it's worth, I use my own router with my NowTV ADSL service (though this is just the usual PPPoA, I personally use the credentials extracted from the supplied router but as I said this is not necessary, it just seemed more 'proper' to me).
A minor note is that unlike Sky, who removed this condition from their terms a few years ago, NowTV still stipulate that you must not use equipment they don't supply to connect to their service (condition 3.8 in
https://web.static.nowtv.com/images/NOWTV_2017/05_Combo/Contracts/NOW%20Broadband%20Contract%20March%202018.pdf). This is not enforced at all, but you may wish to know that, officially, using your own router on NowTV is not permitted.