Whats the best way to setup a webrtc based platform for video call, without worrying about different routers involved?

Question

We are trying to setup a webrtc platform for some medical project which needs to do some video call. One end of the call is on the device terminal that is provided by us and the other end is a terminal owned by the client. Presently we are using only stun servers for the connection and we are having the following outputs :

1. When both the terminals are connected via the same router, non portable, like dlink, buffalo, we get stable connection with no compromise on the video call quality.
2. When both the terminals are connected via a single portable hotspot router, like Jio(India), we get stable connection with some break in between, but usually it connects back. Quality downgrade also happens some time.
3. When both the terminals are connected to a mobile hotspot (mobile phone), stable connection with no quality compromise.
4. When one of the terminals is a router and the other terminal is either a portable hotspot or a mobile hotspot, NO connection is established. The router is an asymmetric router. Although the router is connected via an optical to lan convertor router to connect to the internet.
5. When one of the terminals is connected via a mobile hotspot and the other via a portable router, the connection breaks frequently, but reconnects most of the time. However, quality is often compromised.

We need help into figuring out what should we do to ensure connection stability. What can be done to create a connection in case of the fixed router at one or both terminals. (P.S.: we want to avoid using or setting up a TURN server)

Also, lastly, if there are no alternatives except to setup a TURN server. What should be the best approach to set it up to reduce the cost of providing the service. (Assume here, that we may setup all services by ourselves if required). Cost is the most important criteria, in setting the TURN server.

Thank you all in advance.