There are two classes of SIM card. Pre LTE and the newer LTE type. It should be assumed your carrier is handing out the new version of the SIM. I can not find the numbering on MTN site, but a call in or stop in a store and they should be able to say if you have the new standard of SIM by checking the number on it. I bet you already do.
The 535 does not have LTE and a SIM will not make it work on LTE. The correct radio is missing.
Due to your coverage area you may not be getting the fastest 3G you are capable of as the phone falls back to 2G that shows higher signal strength. When your phone needs to choose data speed often one of the symptoms is quick battery drain.
If you can access the Field Test Menu (it may be locked out on your model) then you could force 3G only.
Open your dialer and call ##3282#
If Field Test does not open you are locked out and can not proceed.
If Field Test does open:
Tap Accept > WCDMA > settings > Under network type choose "3G only"
Now your phone will ignore 2G. However, if you travel outside 3G coverage the phone will have no internet until you switch back to automatic network selection.