Start with what you can see. A lost internet connection should not mean losing your saved posters.
Digital Poster is in pre-release testing, so a screen here and there may not look quite like your build. Setting up for the first time?
Stand near the poster with Bluetooth switched on. In the app, open the poster's Settings and choose Change Wi-Fi network. Pick the network and type its password. Keep the app open until the screen says it is connected. This is also what to do after changing your router or your Wi-Fi password.
The posters already on the wall keep showing, all evening if need be. Only the things that need the outside world pause: searching for new films, and trailers, which skip until the connection is back. There is nothing to fix on the poster itself, and nothing is lost.
Check the poster has power and the screen is on the right HDMI input. Then check your phone: the app needs permission to use Bluetooth and to find devices on your local network, both in your phone's own settings. If the poster is on a different Wi-Fi network from your phone, stand beside it and use Change Wi-Fi network.
Sign in to your existing account first if you can; the sign-in screen can email you a new password. If the account is gone for good, you are still not locked out: install the app, sign in with a new account, and add the poster again while standing beside it. Six digits appear on the screen, you type them in, and the poster moves to your new account with the show intact.
Your picks stay on your phone until you tap Show it, so an edit you have not sent has not reached the poster. If the app could not reach the poster when you sent it, nothing changed on the wall and you can send it again once the app says it is connected. If someone else in the house saved first, the app asks whether to keep yours or use theirs, so nobody's evening gets quietly overwritten.
Tell us what the poster screen shows, what the app says, the step it stopped at and which phone you have. Never send your Wi-Fi password, your account password or the six-digit code. Contact support.