Hello
If you are using wireless, please try with an ethernet cable.
You need to eliminate the basics, as unlikely as you may consider.
Try another computer
Switch your modem off for a few minutes and remove the cable from the wall connector and modem
When your internet is slow, reboot your modem and again during the process remove and reinstall the cable
If none of that does not make any difference, you still have to continue with the basics.
Insist your issue is escalated to a Level 2 technician and tell them you'll wait on the phone, as its rare for them to call you back.
If a Level 2 tech can't see a fault on your line by remote, ask for a proper inspection of your cabinet outside and the street pit for moisture, corrosion, loose connection. Insist, very politely that you want your modem changed.
Finally extract a promise that they will leave your case open
Get that persons Employee D number and a promise they will ring you back after the Level 2 technician has been at your place and the exchange. Ask them to leave a message on their CS system that you will accept a call if they have to ring you back with their D number.
Beyond that, it is only then you can assume it is congestion, and there's your probem. But, you only need a poor physical external connection due to moisture or corrosion, to make a poor speed untenable. Beyond that, that covers almost all issues except congestion