With the Mavic and RC turned in I start the Go app. After a 2-3 seconds the app seems to start turning the screen to landscape mode. Then it crashes. Everytime...
Any ideas?

Please try to refresh the app. If it doesn't help, please try another mobile device to test and see if it is the compatible issue. Sometimes we would recommend using the mobile device on our test list because engineers had tested to run DJI GO/DJI GO 4 on that mobile devices and they performed fine.

Tried with a friend Samsung S8. No problem. Tried it with my Samsung Tab S as well and that works. Well the picture freezes/lags and goes half green on and off, but the connection is there.
Heard of others having no issues with the OnePlus 3T so I thought if it was a setting of some sort.

Have you tried using a cable with a USB connector from the mobile device to the remote?  Others have reported that using a cable that plugs into the remote's USB port (located just under where the mobile device mounts) has solved such problems.

Tried that...

Did you manage to fix it ? I have OnePlus 3T as well and I was planning to Purchase Mavic pro