VLAN on WAN-Port on Fritzbox4020

Hello! I just installed OpenWRT 21.2.1 on my Fritzbox 4020. On Luci switch page there is no column for WAN port:
WAN interface is using eth1 by default.
If I want to put it behind a modem, Deutsch Telekom requires that WAN traffic should be tagged with vlan 7. How can I do that on WAN port, preferably on luci ?
I can set a LAN port to vlan7 tagged and used as WAN with eth0.7, but then a port gets wasted, it would be nicer if I can use the WAN port. I want to configure mwan3, too, so I would like to save ports.
I would appreciate your info!

Check post, no idea about the GUI

According to the screenshot, the Fritzbox 4020 has not been converted to DSA yet and is still using swconfig, so the DSA tutorial does not apply.

Edit the WAN interface, click Device, scroll down the device list to the -- custom -- input field, enter eth1.7.
This assumes that the modem will pass through (does not add or remove) the VLAN 7 tag.

Thank you for your info ! I didn't know that there is such a thing as eth1.xx, it makes sense!