Is this a self install kit?
Most homes don’t have the right signal strengths for the internet and a tech has to come out and correct that.
If you can’t get online with hooking the modem up directly to the first cable coming into the home, you can’t do jack but wait for the tech.
The modem could be provisioned wrong, which happens often with the self install kits and make the modem work here and there but usually very slow when it was working, but they probably checked that over the phone.
To avoid issues in the future, ask the tech to give the modem a cable right off the first splitter and directly to the modem.