Slow Internet Problems

For about the last couple of weeks my internet has been very slow. I’m trying to figure out if it is my Netgear AC-1750 router or the signal coming in from Charter. My house is about 1600 square feet. I am using the modem they provide. I am supposed to be getting 500 mps. The most I ever get is a little less than 300 which is another problem but should be more than enough to not have buffering when streaming or surfing the internet on my iPad. Nothing new has been added to my Wi-Fi that would have caused this. I kill the power to my modem and router once a week to reset them. When I do a speed test it shows that my speed is around 300 mps. Here’s the strange thing. If I plug my computer straight into the modem it shows that I am not connected to the internet and nothing works. If I plug my computer into my router I get the internet. That blows me away because it doesn’t make sense to me. I am going to call charter to come out but I just want to verify that my router is not dragging the WiFi signal down somehow. My tv and firestick or direct wired with Ethernet cables. I just don’t feel like calling charter unless I have to because I don’t want to go through their protocols with killing power and all the crap they make you do. I do that every week already. My Firmware is up to date. I checked it last night. Is there anything else I can do before I have to deal with charter. Why am I having these issues when the Speedtest is showing almost 300 mps. I am not a tech guru but I am not a tech dummy either. Any tips would be greatly appreciated