Off hand, you are likely chocking your WiFi. That makes 20 video streaming feeds at the same time over your WiFi. If you’re not chocking your WiFi, you could also be having the cameras running out of capacity to handle live feeds to five phone at the same time. Try having all five phone watching only a single camera - and make it all the same camera. If the one camera can feed all five phones at the same time, then it is most likely not the camera. If the one same camera can’t feed five live feeds at the same time, then you have found the limitation of the cameras. I doubt most users are trying to watch the same camera from five phones at the same time.
As a test - if you have many cameras - have different phones viewing different cameras. For example have phones 1 & 2 watching cameras 1, 2, 3, & 4; while phones 3 & 4 are watching cameras 4, 6, 7, & 8; and phone 5 is watching cameras 9, 10, 11, & 12. If it is a WiFi problem, the issue will still show up, but if it’s a camera problem, it won’t.