Fails to send video steam but passes all WebRTC Troubleshooter tests

[stevebbs]

I've developed quite an extensive js php webrtc platform for extending the classroom to many online students (using peerjs as the foundation for video, audio & data communication). Several have fast internet connections (e.g. 95 Mb/s both up and down latency 7ms)- even wired through a LAN - but cannot connect their video through the system.

I've spent some time on one of the problem student's (home fibre optic) network using my own laptop (proven to work OK) - using LAN connection to the student's router. The internet is super fast but cannot transmit the video stream (the stream is monitored OK in a video element with window.localStream at the student's transmit end - but not received at the remote end which is on a good working network). My heroku addressing is working fine - the call is received at the remote end and I can monitor that the connection actually makes it into the call.on('stream', function(stream) section of the remote but the video stream fails to show on the remote video element. (In reverse: a stream CAN be received... it is just not successful in sending. Same with data... it can receive data but not send - doesn't get past the var conn = peer.connect(...). I have tried going into their router (I have photos of the different screens) and tried various changes (e.g. taking off the firewall) but nothing helps.

When I test it using https://test.webrtc.org/ everything passes OK (nothing fails at all). Interestingly, though, when I do the same test on my own network at home that DOES transmit the video stream fine, the Reflexive Connectivity consistently shows the following fail: [ INFO ] Gathered candidate of Type: srflx Protocol: udp Address: 110.168.171.250
[ WARN ] Could not connect using reflexive candidates, likely due to the network environment/configuration.
As I say, though... this is the setup that DOES transmit the video stream OK! (mine also fails on the Ipv6 - but it works OK without it). My 4G Android phone (which transmits the video stream fine) passes all the tests (except for Ipv6) - but if I tether it via USB or wifi to my laptop I can access the internet OK through the tethered connection but DOESN'T transmit the video steam - just like the faulty student's machine (and in this configuration it passes all tests - including the Reflexive Connectivity - just not the Ipv6).

Can anyone help me resolve the problem why some are not being able to transmit their video stream, even though their internet is excellent and they pass all the tests in the https://test.webrtc.org test? Plus why my mobile phone on 4G transmits fine but my laptop when tethered to it doesn't transmit. Or at least point me in the right direction. Or if I contact the ISP provider (of the failing student) is there anything in particular that I need to ask them to test or do?