While we couldn't use the old modem anymore, I had an old wireless wifi repeater lying around; after it was configured that did the trick.
Doesn't explain why the new modem doesn't allow it...
Thanks for your help though!
Is anyone familiar with former forum opg.life ? I missed the transition to the new forum over the week end (was scuba diving in the sticks with no internet connection). If anyone in touch with any members please pm me.