Using FRITZ!Box 6690 as a trunk

I have been trying to set up a VoIP solution for our office. We’re located in Europe in case that is relevant. We have two offices that I like to connect to FreePBX which is hosted on a Hetzner cloud.
These offices each have a FRITZ!Box 6690 which is responsible for the internet and our landlines. Our physical phones are connected to this Fritzbox.

The goal is to keep the Fritz!Box and the phones and connect them to Cloud Freebpx. So far I’ve been led to create an IP phone in Fritzbox and use that to create a Trunk in FreePBX but every example I find is with local FreePBX, not a cloud one.
So the question is how to connect these two? Can I run a VPN on the FreePBX and connect the frtizbox to that internet?
And if anyone has experience with similar setups is using the Fritz!Box a good solution or is another hardware needed here?

Bumping this in case someone had a similar experience.