Hello. This is frustrating but I can assure you the cameras do work after a couple of months.
Could your ISP have made any changes? Have you any cameras that are working without issue? I ask because I know some ISPs May block ports required due to the increase in bandwidth usage. Normally takes some time for them to do that.
Worth a check.
It might not be that an it’s hardware issues and if that’s the case if it’s in warranty Yi will replace.