Slow WiFi with multiple modems

We’ve been having WiFi issues. We thought maybe it was our service, which was 25mbps. So we upgraded to 200mbps. But we still had a slow connection and it would take forever to watch something on Netflix. A tech came out and replaced the line from the pole to the house and the modem was getting a strong signal, but the WiFi was still slow and he didn’t know why.

So we went to Best Buy and found someone knowledgeable about modems/routers and asked, and he said if there’s a big discrepancy between the wired and wireless signal speeds, then the router part of the modem/router was bad. Ours was a few years old, so we got a new one (Arris Surfboard SBG10). It was great for over a week, but then WiFi slowed down. An area supervisor came out and it was the same thing, wired in the signal was strong with download speeds at 220mbps but WiFi was only about 50mbps. He said the modem/router we got must have been a bad one.

So we took it back and exchanged it for another one. Now, three days after installing our second new router, we have the same problem. Slow WiFi. I restarted the modem twice, but the speed test still says 6.9mbps standing right next to the modem/router. I find it hard to believe that another router is bad. What might be going on?