Fritz.Box lan to wan to OpenWRT router: How to access the Fritz remotely?


At home I have a Fritz 7590 that is currently acting as Modem/Router/AP/Switch. I bought a Zyxel nwa50ax pro (to be converted with OpenWrt) and I would like to connect the lan of the Fritz to the Zyxel, converting the only lan port of the Zyxel as wan, keeping only the Wifi to access the Zyxel.
On the Zyxel I will configure a Wireguard server, however, how can I access the Fritz 7590 remotely? I checked and the Fritz does not support Vlan, so I am not sure how to do it. I would like to avoid opening any port, setting any VPN on the Fritz or using any other external device (I have limited space for the devices).

Is it feasible somehow?

Thank you.