Who is this FTTH from? Openreach, VM media, Gigaclear or other?
If it is BT Openreach FTTP then the modem is provided and AIUI you cannot swap that out at the moment. Tweaking it/flexibility is not required - the sync speed is the headline speed you sign up for. Full sync at all times. I think that's the case with all FTTH, but I'm not sure.
Whether that translates into full speed throughput is a different matter.
You will need a router, not a modem/router. In the case of OR FTTP it needs an Ethernet WAN port.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59997/15142kbps @ 600m. - BQM