All of the iFly dedicated hardware (700, 720, 740, 740b) is kinda flaky re: wifi when you move them around to different networks, like taking the device from your home wifi to the plane's ADSB device wifi and back home, or trying to connect to an FBO or hotel wifi while traveling, etc.
You kind of have two options: Always delete any previously-stored network credentials (wifi passwords) before trying to log into a different wifi, or else don't try to use the device on multiple networks--let it remember the credentials for a single network (like the one in your plane), and only ever let it attach to that network.
I recommend the second: Just limit yourself to only ever connecting the 740b to you're plane's ADSB wifi network. That's kind of a bummer, but since the iFly devices don't really do well for any use other than as GPS navigators (they're lousy at browsing the web, you can't watch YouTube or Netflix, you can't run Kindle, etc.), and since it's easy to pop the SD card out and update it on a Windows laptop or PC using iFly Connect, I found it really wasn't too hard to live with that limitation.