From experience, the Fritz!Box routers do not do OpenVPN protocol, therefore you can't connect them directly to an Access Server. So your idea to use the Fritz!Box as an OpenVPN client doesn't fly.
You *might* be able to do it with custom firmware Freetz with OpenVPN support, but I don't think I need to tell you that that will likely break your warranty and there's no guarantees it will work.
Best solution to connect a whole network to an Access Server is to set up a Linux gateway client system as described here in the site-to-site setup guide:
https://docs.openvpn.net/connecting/site-to-site-routing-explained-in-detail/
From experience, the Fritz!Box routers support static routes just fine.