How to troubleshoot DSL connection drops?

Question

Computer networks

There are adsl modems - 2 pieces (d-link and asus 2k18).
Sometimes, most often in the evenings after 6-8 hours, the connection breaks. At the same time, there is a dsl signal, but there is no Internet, if there is an auto change in modulation - the speed drops a little and works, but again, not always. Sometimes for whole weeks everything is fine - no breaks. And sometimes in the morning and afternoon cliffs. And all this time, if you pick up the phone - everything is fine - no extraneous noises and sounds.
If you change from asus to dlink sometimes it helps, and sometimes it doesn't. I would be glad for tips in the form of articles, reviews, etc. to get to the bottom of the problem and solve the problem.
ps it is a monopolist.

Answer 1

To begin with, show the signal level on the line when it's good and when everything is bad.
As a bonus, ask to reduce the upload on the port