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!
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day
@BakerMan:
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also...