It took me some time to figure this out, but thanks to @iworx his input I managed to get ipv6 up and running with my Telenet business modem (the modem-only, non-wifi version) with my pfSense router.
The pfSense version I used was 2.2.2.
Here are 2 screenshots of the interface pages. The one thing that fixed it was disabling the “Block Bogon networks” on the WAN interface page, do not forget to uncheck it!
That’s all.