No ISP that I'm aware of will provide troubleshooting for devices they don't own. They just say "sorry, not our device, not our problem". When I installed my own cable modem and router, Comcast was quite clear about that. And I said "fine, no problem".
Most people are unwilling to pay, and yell at customer service. Most of the times, specially when the router has sync it's customer fault.
The problem with this kind of procedure is that it's only a reasonable way to locate the problem when there are problems at that very moment. You're getting stonewalled when - during the day - you're reporting that it frequently loses sync during the night.
You put a customer ID in the tracking system an it queries and stores results. It also performs analysis automatically, but most cases just puts the result in a frontend for analysis.
Out of curiosity, which ISP do you have?
People who don't even know what a router is don't buy their own equipment. Those who buy better routers don't require support for them, they call when there's a problem between the ISP and the router.
I used to install satellite TV and saw this all the time. People would get old receivers from friends and family. Fortunately for me, the receivers were proprietary to that service (we don't use generic pay TV receivers in Canada) and the old ones were built like tanks. If people could have supplied any old cheap generic receiver, I probably would have had a bad time.
I've been a Comcast customer for more than 20 years (in two different states) and have never encountered a problem like this, so I expect such problems are extremely rare. Every issue I've had has been of the kind where Comcast's support person can see right away that there's a problem on their end because they can't even see my cable modem's status even though I confirm to them that it's powered up and the cable is connected (and of course I have to go through the dance of rebooting it multiple times before they'll be satisfied). Most of the time they put me on hold for a while and then come back and the problem is fixed (I assume because some tech in the background rebooted or reset something that was borked). Once they had to send a tech to my house and it turned out there was a bad connection in the junction box they had installed outside.
e.g., AT&T could provide perfect service to the home endpoint, but the customer bought some aftermarket router from their cousin, who had configured it for Verizon. Customer calls AT&T to holler. Tier 1 support doesn't know what that particular router config GUI even looks like, so it gets bumped to T2 or T3. Ultimately to find out that the customer's cousin had hardcoded DNS to some internal Verizon system that's not visible to AT&T.
Repeat x100K. ISPs job isn't just "provide the Internet," it's also "provide all the troubleshooting for every non-technical customer who just wants to watch Netflix but doesn't even know what a router is"