Long story short, it seems MTN sent a batch of SIM cards to Cool Ideas that are giving issues (including the SIM I received). Cool Ideas managed to find a SIM from an old batch that still works fine, and they are couriering it to me.@Chris_Auc: I hope you can get this issue resolved very soon !
Can you communicate with the SIM card from the USSD menu of the G5TS, can it receive SMSs, make calls etc. ? Can you read the phone number from the Device Information menu of the G5TS ?
I would try to communicate with the SIM in the hope that something starts working !
Will report back once I receive the new SIM.

