Poor service.

Up to December 2019 I had a stable 3G signal at my house, since January I have received an very erratic SLOW [0,5 mbps] service. I have sent several complaints and service requests. They send an SMS saying the complaint has been registered, a few days latter I get an SMS saying the problem has been repaired, but they have done nothing. I bought a pay-as-you-go data sim from another provider.. inserted that in the router and get between 4 and 5 mbps down load. Does anyone know how I can get help with this!!!!