I had a TP-Link Extender with my Mweb router working for months. Mtn installed a Cel-Fi signal booster to boost cellphone reception. With Cel-Fi booster switched on cellphone signal is 4 bars, but it kills TP-Link wi-fi.
It shouldn't be any interference, I suspect that booster in not installed properly.
Report it to MTN and insist to sort it out, as when booster works unstable, it can generate lot of radiation in wide spectrum.