I have a different router from Asus but probably the same firmware. When I enter everything like this, I always get the error message "no valid IP address" for the IPV6 prefix. Any idea what could be the reason for this?

I know that my question is not about an Asus device, but I assume that the information is quite identical and since the topic here is going in the right direction, I am happy to ask my question here.
I don't quite understand it either, I use a Netgear RAX80 router as my main device, I don't have a Wingo modem any more either. If I now select 6rd Tunnel, the good guy charges me relatively much:
6rd prefix: think here comes 2a02:21b0
6rd prefix length: think here 32
6rd IPv4 frame relay address: no idea
6rd IPv4 address mask length: no idea
IPv6 DNS server: Auto? If no which one?
Secondary DNS server: If not auto which one?
Use this interface ID: If yes, which one?
I really can't figure it out, I already contacted Wingo and got the feedback that such DNS settings are not possible via Wingo. However, there is only this entry under advanced setup/IPV6 and I assume that the configuration should already be carried out here. I also have the whole thing as a print screen, in case someone is bored and is good at explaining :D

Yes, IPv6 is supported according to the Wingo website with "For IPv6, you can configure 6RD using 6rd.wingo.ch as the gateway, 2a02:21b0::/32 as the IPv6 network and a bit mask length IPv4 of 0. The MTU is 1480 bytes" supported. This must be activated and then IPv6 is supported if the Asus router is running in Access Point (AP) mode. I doubt whether it also runs in router mode with the "Passthrough" setting.

Please proceed as follows: 1. activate iPV6 in the WingoBox; 2. operate the Asus RT-AX88U in Access Point (AP) mode and then it will already work. If you want to operate the Asus RT-AX88U as an additional router, iPV6 should be set to "Pasthrough" (no additional settings required), then it will also work.