Can you connect if you put the Slate AX as a Client to the Flint as WG Server locally?
Flint (WG Server) LAN → Slate AX (WG Client) WAN. Update the Client settings (‘endpoint’) to reflect the Flint’s LAN IPv4 (also an endpoint, normally 192.168.8.1).
… & don’t forget to change the Slate AX to a different subnet like 192.168.18.1 (GL GUI → Network → LAN → Private Network → Router IP Address).
wg show is a quick way to confirm connectivity if you can SSH. See below: