Ok so :
running OpenVPN client on the Slate, no IPv6
running OpenVPN Server on the RT-AX86U, IPv6 enabled.

Trace route below. I stopped after 9… it goes to 20.

HTH
k.

PS changed Slate to IP 192.168.7.1 as 192.168.9.1 is the Guest Network. 192.168.8.1 reserved for fixed MiFis.