Just test out the router, and then you’ll be able to confirm who is correct. I’ve been in the exact same situation where I was adamant it wasn’t something I changed, gave them the benefit of the doubt, and let them install their branded one. The internet issue continued and it turns out I was right. They configured something via their end, and the router started working. I then switched back to my old router, it was getting the full gigabit, so I was correct.
Just test out the router, and then you’ll be able to confirm who is correct. I’ve been in the exact same situation where I was adamant it wasn’t something I changed, gave them the benefit of the doubt, and let them install their branded one. The internet issue continued and it turns out I was right. They configured something via their end, and the router started working. I then switched back to my old router, it was getting the full gigabit, so I was correct.