@sky75 they have provided to me some beta build which fixed the issue. :)

@Sunshine Hello the ISP is A1 Bulgaria and the Firmware version is: 1.00 0.91 v0001.0 Build 220307 Rel. 60743n. This is the test scenario: 1. turn on the router 2. tether app via mobile network is...

I have pretty the same issue using MR600 v3. If I reboot the router the tether remote login is working for 5 minutes - than the device is offline - but I can see all connected devices to the router...