Hey again Valdravannia
It definitely sounds like something with the network, although I’m not certain if it’s the modem or router. What is already pointed out regarding 2.4 and 5gHz may be worth checking out along with just trying a wired connection for testing purposes.
The modem itself though did bring up some results and may explain the symptoms and test results. The Arris Surfboard SB6190 modem uses an Intel Puma 6 chipset. There’s reports of issues with those regarding their performance. In some cases, just getting a firmware update will fix it. In others, a replacement was needed with the goal of getting a non-Puma chipset in it.
A Google search for “arris surfboard sb6190 puma chipset” will likely return some information about the situation.
The firmware update does tend to fix those usually and since you own the device, you’ll want to check the manual or contact the manufacturer for information on how to update it. Beyond that helping though, testing a different modem or purchasing a different one may be the fix you’re needing here.
Edit: I noticed your last edit as I posted this so I’m mostly just verifying your information with some extra info to help if needed.