Someone needs to explain why it took 18 months to figure this out considering how simple it was to detect. You have a problem that's repeatable right on schedule, that presumably shows up in the logs and affects multiple customers. It should be a no-brainer to send someone with test equipment out there to measure things in the field while the problem was happening.
1. You have an issue with your broadband in a rural area. You call up your ISP. You get some handwavy answer about contention and they ask you to restart your router and call them back if it happens again.
2. The next time they agree to have an engineer take a look. You hear nothing back. You contact them again, they say their engineer looked at the exchange and found no issues.
3. The next time you ring them and they agree to send an engineer to your house. The engineer (contracted out) arrives at 2pm, they check your modem, rant a bit off the record about how they don't fund shit, then determines that everything's working fine at 2pm, and it's the ISP's problem that their network collapses at 6pm because they oversold the rate of your village.
4. You contact the ISP. They read the engineer's report, saw it was working fine at 2pm, and decided they didn't need to take action. They suggest using the internet at another time of day, such as when you and most of your neighbours are away from the house and at work.
5. If enough people in the area decide they complain they might finally deign to do something about their service in a few months. More likely is some other ISP comes around and signs up all the customers so pissed off with the last one, until most of the village is now on this ISP and the same problems reocur.
6. After like 4 or so years you've looped through all the ISPs, maybe there's a new generation of tech which promises higher speeds but they can't be bothered funding anything locally so your actual speed and reliability is unchanged.