Hello,
I have a rough understanding of your network topology diagram.
It may not be possible to implement such a WG Client - WG Server - Zerotier link since there is no corresponding route on the Client router and Server router.
If a new route is manually added, it may cause WAN (Internet) exception.
If you want to access resources from Zerotier, you should also enable Zeroter on the Client router and add the same Network ID.