Unfortunatly the antenna for the device is inside so that prvents you from getting a third party replacement antenna to provide more signal strength.
As I see it you have two options and someone may chip in ith a third or fourth if they come up with something different.
1. You can replace the DSL modem router with one that you buy and connect yourself instead of taking the one from AT&T. That way you can research and get a better brand or model that will give you more signal strength.
2. If your service is fast enough that you can tell that the wireless signal is being limited by the antenna of the 7750 then you could purchase a wireless router and connect it to the 7750 by cable and use the wireless function of the new router.
To determine what your download and upload speeds are connect a computer by cable to the 7750 and run the speed test and write down the results. Then take and run the speed test using the wireless from different distances from the 7750 to see the drop off and range that you have. Then you can decide which way is best for you, get a new modem/router or add a wireless router.