I have been running an MX with T-Mobile Home Internet for the last 3 years in a dual carrier configuration with a cable provider.  It is working, including  using 5G only with the cable modem disconnected for weeks at a time. I set my TMHI connection as the primary so the connection so SDWAN VPN's to other locations (including other TMHI connected MX & Z1 )  is primary over this interface . TMHI is native IPv6 in their backbone.  The premises gateways do a NAT46 and then as it exits the network, there is an additional  NAT64 within their network. This is a performance bottleneck and a likely source of issues with the VPN.  Its possible that in your region this may be causing your issues. I doubt this will solve your problems, but you may want to try overriding the default DNS on the Z3 WAN interface and set a static IP+DNS since it is just a private subnet between the Z3 and the gateway.  I would also set a preferred DNS provider for the client vlan as well if you have not already.  ... View more