I have had many problems with losing contact with my IOT´s especially cameras. I had to delete them in the mobile-app and then adding them again, to re-establish the connection.
It seems that using dynamic IP´s is the cause. The server – usually Your router – sets a time when to renew the IP-adress it assigned a client when it connects with “dynamic IP”, until then it will connect with the same IP for a client, like a laptop, a TV-box or an IOT. Some clients binds via the router to some remote server when installed, like when using the Nedis or Tuya-app. So when the router assigns a new IP dynamically after the decided time with a specific IP, the remote server will not find it again.
The remedy is to use “fixed IP” instead of “dynamic IP” for clients that have problems of being “lost”.
Try this link for an explanation on fixed vs dynamic IP.
I had problems especially with a range extender working in between the router and the cameras. By entering the range extenders setup and changing to a fixed IP, the problems disappeared. But using fixed IP´s is a good remedy whenever You are loosing the connections with Your clients (things in Your network).