In my experience with multiple E6330 and one E6430S which is close enough to 6430, a clean install of Windows 10 OS never has any issue w wireless.  I suspect because you switched wireless on a previously installed OS that has old driver, it may have caused some messy driver conflict.  I think before you give up on the new wlan (for E7240), try a clean install of Windows 10 and let Microsoft automatically find the best driver for the new wlan.  A clean install imo is better than user messing around later downloading Dell or vendor driver.  You can try this if you do not mind backing up data then wiping the hdd.  If a clean install still results in Wi-Fi crash then you can choose the Dell oem dual band wlan validated for this model E6430.
Hey, I bought a Dell Latitude E6430 a few years back and even though it is slow compared to modern day laptops it's fast enough for me. Only problem for me was always the WiFi, I wanted to upgrade the WiFi card so I wouldn't be so dependant on using ethernet. I decided to buy a 'Intel Dual Band Wireless-AC 7260HMW' WiFi card. This has 2.4ghz and 5ghz which I assume is good, also it has Bluetooth.. I think.. so I got it, put it in my laptop and tried it without drivers, my WiFi speed went up from 50mbps to around 150, I tried it again after downloading the drivers and got 255, which is probably as high as it will go as I'm 2room away from my router and my WiFi goes up to 300. So I tried gaming with it to test it properly and it was working fine.. until it wasn't. It would make my WiFi crash every few minutes so that I would have to sit and wait like 3minuted for it to work again. I need help with this, it seemed to work but maybe it's too powerful for the laptop?? I did this with a fairly powerful charger attached tho so I feel like that shouldn't be the reason, someone help.. please.
Upgrading Dell Latitude E6430 WiFi Card to Dual Band