In case it might help someone:
I bought two Shelly Buttons which came with firmware 20201210-145729/?@47dcf392/. After connecting to button WiFi AP and entering my home WiFi name and password under the client option, the button would claim to connect to it and that it would be reachable on the new IP address but I was unable to connect back to it (I found its new IP in my router web interface under the client list). There might be an issue with WiFi 6 dual band routers (I own a Asus RT-AX86U) or just a bug in the firmware, it's hard to say... (Edit: It seems this issue occurs on other non-WiFi 6 routers, too! 
)
What solved my issue:
I used the reset procedure (open back cover, hold reset button under battery,...), then used the USB cable to charge the button which puts it in an online state, then connected the button to my Android phone hotspot WiFi as client instead of my Asus router. Because my Pixel 5 phone does not show hotspot client IPs, I then connected my laptop to the hotspot as well and used the free "Advanced IP Scanner" (Google it) to find the IP address of the Shelly button. Then I was able to perform a firmware update of the button and afterwards set the WiFi client back to my Asus SSID and password and THEN it started to work like it should have from the very beginning 
Edit: For an iPhone hotspot, you can use the "Network Analyzer" app from the App Store to get the button IP.
