First, please mind that a residential/mobile IP are real users' devices' IPs, and therefore can be used only when the user's device is idle (i.e. the device is connected to the internet, has enough battery power, and the user is not currently using it).
If the IP becomes unavailable, our system will automatically assign you with another available residential IP of the same type and geo-location as you used.
Setting Long Single Session when using Proxy Manager
Choose "Long Single Session" on the port settings drop-down, and it will automatically ping the IP to keep the session alive: