OP here again. Thanks for the contributions. I have dug out the OR modem - it is a Huawei HG612 3B, which I believe is a VDSL2 modem.
The village in Germany is on a new FTTC install, with the cabinet very close to our house. We should get a very large fraction of the 50/10 product we have paid for.
As far as I can tell, I should be able to plug the the modem straight into the new VDSL2 faceplate, then connect it to my network of Hub 5 routers. They are currently all in AP mode, hanging off a Netgear Trek PR2000 (recommended to me on here) which connects to the neighbours' wifi (we pay them). I will disconnect the Trek and turn the router of one Hub 5 back on and set it up with the login details and settings for the new service.
After that, Robert wird mein Onkel sein!