Network unstable problem

Hi,

I have 4 cameras connect via ip, using the hik vision app on my iPhone iOS 13.1.3

Sometimes I can only get one cam working, other times none.
They keep saying ‘network is unstable connecting again’

It says my device is offline

any ideas how to fix this?
There always seems to be a problem with these cams, had them for about a year and it’s rare all 4 work.