How to fix connection problem or invalid mmi code error on samsung galaxy android mobile U

This video shows that how to fix connection problem or invalid mmi code on android phone.these type of errors are occuring due to the sim card management .
How to Fix Connection problem or invalid MMI code error occurs in any mobile phone having any Network Providers (SIM). Are you getting the message .
For instant recharge, i am dialing *120*16 digit pin number from the recharge coupon or voucher#. At this time ussd code starts running and displays .
How to fix Running USSD code error Connection problem or invalid MMI code in Any phone model like Samsung Galaxy Note 4, Sony xperia, Moto g, .