why is the zlt s50 router really slow

For the past 2 years, my family has been looking for a good Wi-Fi. We've used numerous devices from different companies. DSTV's router was limited to the available data but had pretty decent speed. Huawei was incredible, when it decided to work and TP link, which is the most recent one we used prior to our current router, was the best. Great speed and massive available data (that probably depends more on your usage). but the issue with TP link and the reason for disconnecting with it is because they changed service providers and cut our internet for a month that we paid for. They changed from Telkom to MTN, yet we requested them for MTN from the beginning of the contract and said they only use Telkom. We couldn't trust them anymore. But the biggest disappointment is the MTN ZLT S50 Router, really slow and connectivity issues are very frequent. We place it where we place the previous routers and no change. we place by the window and it's as if you hadn't done anything to it. connect the ethernet cable, still nothing. Try watching a 5-minute video on YouTube, you'll be halfway done in with it in an hour, we adjust the antenna and still the same results. it agonisingly slow and borderline unusable but if there is something worth doing to improve the speed, please let me know cause we also updated and still nothing different happened