It wasnt obvious to me but when I had to change my C310 originally set up for cable back to wifi, you have to use the mobile you first set up your TP link ID with at the very beginning.  You will see that the Tapo app is subtly different from the one you will see on all your other subsidiary devices

you cant eg add a new cam from other phones/tablets. it wont be found, no explanation or hints

so go back to your first phone and its all ok

I guess its a security matter they dont choose to tell you about - like so many things

I suppose we will eventually have a list of tricks a mile long

but on the positive side I looked at semi pro ptz cams of equiv 1080p spec to the C200 you could pay £500 - £1000 each!!!!!!!!!!!!!  C200 ca £30 nuff said world beater IMHO