This is the same issue many people have that I struggled with for close to two hours and finally figured it out, hope this helps...
I have the first Aria, never had issues reconnecting after changing batteries until today. Phone will not connect to Aria, Aria would not connect to Wifi. I have tried everything in order to make it work.
So I'm not sure if it's one or the other or the combination of both but first I deleted the Fitbit app, reinstalled the app, logged back in and then deleted Aria on the app from my connected devices. Secondly, I switched over to Google to sign in (which will be the new and only way to log into Fitbit starting next year), agreed to all the mandatory checks allowing google and fitbit to collect all of our data (which I believe we should get paid to provide).
Once you are signed in using Google, try going through the whole process again, but this time you are adding a new device to your app, and all of a sudden things start working step by step, create name for your Aria, add your initials, put the Aria in set up mode by putting back the new batteries again, connect the phone to Aria network, click connected *(do not wait until Wifi says it is connected but just press connected anyway), if it doesn't connect right away then you will be brought to another page to click on your Wifi and password, then go back to your Wifi and connect, click on connected, then finally you will see a check mark on your Aria, close your app and reopen then you will see your added Aria!
Good luck everyone, hopefully this saves you some of your valuable time and not waste two hours like me.
