The same thing happened to me. Original a master reset and toggle aeroplane mode on switching phone off back on and turning mode off again seemed to resolve it. Problem is about 3 days later it happened again. This time it wouldnt fix itself. Very fraustrating indeed. However I think ive found a solution for those with an unlocked phone like mine. You may need to reset the phone first. However possibly not. I did.
Put a sim card in from a different provider. Doing this appears to force the phone into a full network reset including aeroplane mode. You should recieve signal on the other providers sim, put yours back in and you should now have signal.
I hope this saves a lot of you the fraustration of this situation and a working A71.
Let me know if it works for you in the comments
