I fully agree that you need to use simple ping commands to test this. The first level ISP tech you talk to might not even know what a ping command is. You show them something like winmtr and they will recommend you reboot your pc or reinstall windows to fix it
So if we believe what winmtr shows there is a issue with the connection coming to your house . This is good in many ways since the level 1 tech guys have magic buttons (ie tools they don't understand) that will check the line to your house and if they see errors they will send out a tech.
The main step is to get them past the steps in their scripts where they blame your equipment. You want to have 2 constant pings to show them. The first to your router and a second ping running at the same time to their router, likely 10.15.80.1
My complete guess is that you have some issue with physical wires going to your house. What you show is just data loss but you do not also have large latency spikes. If you have large latency spikes and loss it could be a overloaded connection but just loss tends to be some kind of hardware error.