"no tethering device found" on GL-750C6

Hello friends,
After updating the router firmware (current v.4.0) I’ve discovered that the mobile connection is gone - router won’t read SIM and tethering is not working. So far I’ve tried resetting the router, reseating the sim, making the full power cycle, no luck. Any suggestions on how to resolve this? Kirill.

Can you try reset the firmware using the reset button?

Thanks for the reply; I did, no luck