The iFly-dedicated devices are notoriously finicky regarding their wifi behavior. They're not like a laptop or tablet device, where you can set the credentials for a network and log in, then carry it off to a different network and set those credentials and log in, then come back to the first and have it remember the original credentials and just automatically log in, and then let you go back and forth and automatically log in using the appropriate credentials.
Why it doesn't "just work" like that, I dunno...but it doesn't.
As long as you only have one set of credentials saved for one specific network, then any time you power up the device in range of that network it will reliably log in and connect. When you have more that one "remembered" network, then all bets are off and you can expect troubles like you're describing.
I used an iFly 720 for years with good results once I figured that behavior out. Instead of trying to connect it to my home network or a hotel network for updates, I just pulled the SD card out and updated it via my laptop using the iFly Connect tool. That was faster than doing the updates directly from the 720 over wifi anyway.
Alternately, you could try deleting the credentials from the last network you used before trying to log into a "new" one...so, delete your Stratux credentials before logging in at home, and then delete your home credentials before re-logging into your Stratux the next time you fly. But that may also be more trouble than it's worth. I've never owned a 740, but from monitoring others' experiences here, my understanding is that it's not always straightforward to delete credentials from a previous login. Maybe Brolin or another 740 user can share the details for how to do that. (Networks with stored credentials show up with a "(Pre)" as part of the name, but I think maybe you can only see them briefly after a powerup and they'll eventually disappear if they're not currently in range...?)
I really think the simplest option is to just set it up to connect to your Stratux, and then leave it alone and pull the SD card when you need to update it.