Fritzboxes are very special. If they are not in “fullbridge” Mode thay internally add the vlan ID.
There are two ways to connect with a VLAN.
If the moden internally add a vlan ID in the IPFire must choosen PPPoE and no VLAN.
In this case you have to reconfigure the Modem from VLAN 7 to 40 to switch to the MNet connection.
If the modem not add the ID in the IPFire must coosen VDSL and VLAN7 or 40 depending from the ISP.
Also keep in mind that a created VLAN device cannot removed. If you change this in the GUI you have to reboot to remove the VLAN Interface.