Hi,
You can choose Priority based load balancing method in traffic manager.
When it comes to health probe configuration, you can either choose HTTP or HTTPS or TCP.
You cannot choose TCP because App GW will always listen on the port of your service.
You will need to go with HTTP/HTTPS based probe, where if the service returns you a non 200 OK response and it crosses the threshold then, Traffic Manager will start switching to the secondary endpoint.
When both endpoint receives non 200 OK response, then TM will resolve to both the IPs. So client can actual sent the request to the server and server returns the error.
Regards,
Karthik Srinivas