Question Download speed slows after modem/router reset

[sketchyguy]

I'm on Cox and paying for 50 Mbps down and 3 Mbps up. For the past couple of weeks, I've been noticing slow internet and when I run a speed test, I see that I'm only getting 2-5 Mbps down. The upload speed is always steady at 3 Mbps. I've noticed that when if I reset the modem and router, a speed test will show speeds up to 60 Mbps down. However, this only lasts for about 10 minutes before speeds go back down . I've tried calling Cox support and having them reset the modem from their end, and the same thing happens (speed is good for 10 minutes, then slows down.) Today a Cox technician came over and tested the signal, from the wall, showing that it's steady. The same goes from the signal coming out from the modem, using the same ethernet cable that I use to connect to the router. I then tried connecting my PC directly to the modem, and the speed remains steady at 50 Mbps then.
It would seem like the problem is the router, so I went and bought a new router. I just installed it and it looked like everything was good, but then again after 10 minutes the download speed went back down to 2-5 Mbps. Any ideas for what could be wrong? My only thought is that I could get a modem-router combo since something is happening between the modem and router, but I'm not sure. The signal from the modem is stable, the ethernet cord between the modem and router has been tested to work, and the router I'm using is brand new.

Modem: Netgear CM400 (I got this earlier this year)
Router: Tenda N300 (Just bought this new today)

 

[Ralston18]

How are you physically connecting modem, router, PC, etc.?

This router?

https://www.manualslib.com/manual/943811/Tenda-N300.html?page=14#manual

See physically numbered Page 1 (Chapter 3).

Do you have another splitter (filter) available. Splitters are cheaply made and can often cause problems.

Try another phone cable between splitter and router.

 

[sketchyguy]

How are you physically connecting modem, router, PC, etc.?

This router?

https://www.manualslib.com/manual/943811/Tenda-N300.html?page=14#manual

See physically numbered Page 1 (Chapter 3).

Do you have another splitter (filter) available. Splitters are cheaply made and can often cause problems.

Try another phone cable between splitter and router.

Thanks for the reply.

I have the coax cable from the wall connected to the modem. I have an ethernet cable running from the modem to the router. From the router, I have an ethernet cable running to my PC. My setup looks exactly like the picture you posted, except I don't have another phone connected to the modem. I am not using a splitter, everything is connected to the router itself. I do have 3 things total connected to the router via ethernet cable (my PC, another PC, and an Ooma Telo, which is a home phone that works through my internet). Do you think a splitter would help?
One thing I should note is that my speeds with the new router have stabilized to around 15-20 Mbps, which is a significant improvement from 5 Mbps, but still not 50 Mbps.