There are a few to fix Connection Problem or Invalid MMI Code on android.
The simplest way to do so would be to restart you device. This should work most of the time.
If that doesn't work go into Settings -> Network Connection -> Mobile Networks -> Network Operators and select Wireless Provider during the search. You may have to repeat this a few times before you will connect to your network.
If that doesn't work try activating the Radio and Turn on IMS over SMS. To do this enter service mode by keying in *#*#4636#*#* in to the dialing pad. You don't have to press send as it will automatically enter service mode. Go into Device Information or Phone Information and select Run Ping test. Then select Turn Off Radio, restart and reboot.
- Jake Lam
The simplest way to do so would be to restart you device. This should work most of the time.
If that doesn't work go into Settings -> Network Connection -> Mobile Networks -> Network Operators and select Wireless Provider during the search. You may have to repeat this a few times before you will connect to your network.
If that doesn't work try activating the Radio and Turn on IMS over SMS. To do this enter service mode by keying in *#*#4636#*#* in to the dialing pad. You don't have to press send as it will automatically enter service mode. Go into Device Information or Phone Information and select Run Ping test. Then select Turn Off Radio, restart and reboot.
- Jake Lam