With the 3Gps service you get a separate ONT which is screwed to wall and separate Technicolour router ( there is no modem as such with FTTP, so nothing to put into modem mode like you do with Virgin). So yes you can use you own router by replacing the supplied one. If your intension is to get better wifi is a different location to where the router is, then what you need to buy is an access point (WiFi 7 if you want to make the most of your 3Gbs service and have compatible clients), not another router. I can a little over 2.1Gbs via a UniFi U7pro access point using 320MHz channel width.
An access point just does wifi ( no routing, no modem, no switch).