I had a good deal of trouble too after we installed a new router. This and more.
Use the supplied screwdriver..small screw very difficult to remove to get the case off.
Press side button and release it very quickly to get into setup mode.
Keep your phone or device very close to the doorbell and wait for the ring network to appear in your list of networks, it takes quite a few seconds.
It would not easily connect to my router. Instruction to move bell closer to router impractical when bell already installed. Eventually worked when I stayed close to the bell with the front door open and typed in the router password there.
About 2 hours to get it working.