Every night slow 5G speed

We started with a 30 meg line, all was well and then suddenly at 19:30 or just pass 20:00 when the call center was closed our line speed would drop to just under a meg and we cant’ use the wifi till after 21:00. We upgraded to a 60 meg line with the hope that things would be better. It started off promising but then suddenly slow speed again. Dropping to between 5 and under a 1 meg line after 19:30. I have phoned a couple of times to report the problem but every time I get the same answer. They can’t see the problem and it's because it’s peak time or there is a problem with the tower. Last night I spoke to a very friendly call center agent. She said she would escalate the problem to the technical department to investigate why only at peak times does our line drop to under 5 meg. Now my question is why does our line drop from 60 meg to under 5meg or 1meg. I pay every month for a full service not to only use my wifi between certain times. I have asked before for a technician to contact me to explain why this is happening and nothing. Can this problem be addressed and corrected? I have to work at times, and that I can’t even do it because the speed is too slow. I need to know, will this be sorted out or I’m going to sit with this issue every night because if this is the case I need to start looking for a different service provider as I can’t see why I must pay R780 a month for a service I can’t use during peak time.