Internet speed inconsistency

One day my wifi would be fast, the other itll be slow and its been like this consistantly ever since i upgraded to the new router. I really dont like the new router and want an old one because it has way less issues, its unfair that the new router was forced on me and i gotta pay for a service that ISNT meeting the performance expectations i paid for. I even tried disconnecting all devices, restarting them and the router itself. The cables were fine and stable it just makes no sense. It doesnt help that the customer service and the last person they sent out here was practically useless and wasted my time when i needed them to resolve an issue and they just abandoned and rescheduled without communicating with me. All they said was "ill be back im gonna go grab my ladder" then 10 minutes later i get a text saying "we're sorry we missed you, click this link to reschedule" WHEN i let him in to take a look at the issue.