I agree with the poster that people don’t know networking. MTU default is 1500 and should be left alone. I used to set jumbo frames on our enterprise network which is setting MTU to 9000, and this can improve performance.
Use AT&T’s modem then buy your own mesh network from there. My house uses 4 routers so I can get 900Mb from all 5ghz devices anywhere in the house. Plus, if you get mesh routers, always get the wifi 6e or newer routers because of their dedicated backhaul used to talk between each router if they are not wired. Then from each router, I use either the ports on the routers or hook up a 5 or 8 port switch so I can hook up each device by Ethernet, I never use wifi unless if I can’t go wired.
1 more tip, I use a commercial switch in my audio room and it improved the sound quality because this switch doesn’t use a wall wart, and instead of going out of the router to the streamer, I go thru this switch and it improved the sq.
Only use wifi if you can’t use a wired connection.