VDSL2 Modem
1 x 100Mbps LAN
1 x unused LAN
That actually fits with something I found, but it was just a blog-type page so I didn't take it as gospel. Looks like it's only 100Mbps then... which should be OK really.
After all, the modem is basically just converting from set of voltage signals to another (OK, a bit of a simplification

) so it should be able to get pretty close to the 100Mbps without trouble.
Whereas a router has rather more work to do... I know a few people have found increased speed on FTTC by switching from a router with a 10/100 WAN port to a gigabit one.
Would that be something to do with the port itself, or the likelihood that a gigabit router would have a faster processor?