Not sure why you would want to move over from the best network to the worst, but that's your choice.
When you called MTN, did they give a reason as to why the router was locked? Some routers you can get an unlock code for, but there has to be a reason why this specific router is network locked- is it a Fixed LTE package that you signed up for to get this router, that would lock the router to your physical address? Is it one of the MFR289 router?
Did you buy the router or did you take it out on contract and your contract has ended? Sorry for all the questions, but just need to understand.