I use a TCP OpenVPN connection both-ways between two GL.iNet routers, and mobile devices can access the Tablo v4 on the other end just like your setup. I’m thinking it’s because the mobile client setup uses the account login process so it might be getting the Tablo IP address from the cloud.
And, like you are experiencing, my Fire TV device didn’t connect. I’m thinking that because it doesn’t do the same account login process as the mobile app(s), it is probably using a network broadcast to get a response from the Tablo on the network to obtain its local IP address in order to connect. So, when I enabled IGMP Snooping on both GL.iNet routers, the Fire TV was then able to connect to the Tablo on the other side of the VPN.
My Internet connections use the same commercial service provider on both ends with 450mbps downstream and 20mbps upstream. So it’s effectively just 20mbps and I’m able to stream from the Tablo just fine between the networks.