iPhone tethering failing

I have had the mango device for a couple of weeks and tethering has been working fine. But a few hours ago there would be no internet connection when tethered. It only works if I reboot the mango device but only for 30 seconds at a time. I haven’t done anything differently from previous times it has worked

Can you let me know when this happens, the tethering is still there and you have IP address?

Can you get the log from the router when this happens?

Yes the tethering is still there and yes I’m logged in.

The tethering is still there but in the Interface it displays “no Internet” right?

Before reboot, are you able to login the router and get the system log?