Slow Internet speed

I don't know where to go about my slow internet speed.  My plan is 2100 mbps but at night my speed drops to between 6 and 30 mbps and nobody is fixing it.  This has been going on since November 2024 and all Xfinity does is send a home tech to come check the connection in my house.  Since the problem started everything in the junction box in front of my house has been changed out and a new line has been installed.  Every time I try to talk to a live agent the Xfinity Assistant stone walls me and just wants to keep restarting the modem.  My bill never gets adjusted for not getting the service I pay for and when I do complain enough the give me a small fraction of a credit.  I'm just about done with this hassle.