@Schoflemen79
thanks for contribution and the feedback
to complete the answer / solution @Doremi
this recent discussion can help
Report on good VDSL2 experience with Zyxel XMG3927… | Swisscom Community
post @lucaberta 27.11.2020 17:03
"… The IPv4 Mask Length is 0, which means that the whole IPv4 address (32 bits) is mapped in the IPv6 address.
The Service Provider IPv6 Prefix is used as the main supernet in which 6RD maps the IPv4 32 bits plus the local interface addresses…. "
also in the german Community
XMG3927-B50A u. Wingo?! | Swisscom Community
the first post 17.03.2020 22:50
“… Für IPv6 kannst du 6RD konfigurieren, indem du 6rd.wingo.ch als Gateway, 2a02:21b0::/32 als IPv6-Netzwerk und eine Netzmaske (bit mask length) IPv4 von 0 verwendest. Die MTU ist 1480 Bytes…. ”
“… You can configure 6RD for IPv6 by using 6rd.wingo.ch as gateway, 2a02: 21b0:: / 32 as IPv6 network and a network mask (bit mask length) IPv4 of 0. The MTU is 1480 bytes…. ”