Nighthawk gateway choices

I have TMHI in rural Florida. I had many issues with the Nokia and Arcadyan gateways they supplied, but when they worked, typically got DL speeds of 100 to 300(or more). I got a Netgear MR6500 and so far am getting between 6 and 90. This is with a laptop 6 feet away. The gw has 4 bars in the best location. I am considering getting the Netgear M6 antenna, 600451. ($55 on Amazon). Is this the best for that price range? Is it likely to improve my speed? The tower is 4 or 5 miles away. (I will be using a TP-Link Archer AX21 WiFi 6 Router to connect to.)  It's att branded, but it's unlocked,  GSM and compatible wit T-Mobile. I now suspect that it is lacking some bands. Specs show different bands.