Hey,
thanks a lot for the suggestion. It worked and I now have full internet access with my Flint 2 Router via my ONT device.
I had to install the pkg via SSH, but still connected via PPoE. It auto created the DSLite dual and WAN6_4 DHCPv6 interfaces needed for AFTR (would only work automatic since I had no AFTR-credentials).
The only downside is, that the GUI still shows that I dont have Internet connection, which tbh. isn't a big deal.
Thanks for anyone who contributed to this thread and helped me find the solution.