My Internet is constantly disconnecting

Does anyone else have an issue with the Wi-Fi constantly disconnecting from devices? I had the Internet 1000 plan and before that, I had Internet 300 and before that, I had Internet 100. I've had issues with my Wi-Fi disconnecting with each different plan. After getting Internet 1000, I used to get lower speeds than I did with Internet 300. I've had a technician come in to try and fix it but he only suggested I buy a Netgear router and while spending over $280 for a router helped, it didn't help by much because the modem still doesn't support speeds higher than 600 mpbs. It worked decent but recently, I've been experiencing my Wi-Fi disconnecting constantly on my phone and other devices. Does anyone know why? Any fixes?

Also, I was thinking of purchasing a modem that is compatible with AT&T and also allows me to use the 1 GBps to it's full potential but I can't find any. Does anyone know of a modem I could use? Anything that performs better than the modem+router combination equipment AT&T provided?