The issue is not BI. BI is just exposing the problem.
That is likely your problem and where investigation needs to focus, at least initially.These are all on Wi-Fi....
What changed 2 weeks ago that could have adversely affected your network, particularly the Wi-Fi?They were working fine for months, then this started about 2 weeks ago.
Check everything starting at your router onward, including your dhcp functionality and all your IP address assignments for any oddball randomization.
Double check any/all access points or extenders for proper configuration and operation. A simple power interruption can make a mess of that stuff requiring controlled reboots or re-configs. IP addresses in particular can get scrambled behind access points and bridges under shaky powering conditions.
Try accessing the cameras directly via their webUI while Blue Iris is reporting a 'no signal' state to see if maybe they're actually online, but just not being seen by BI. If they are, BI is possibly changing the ip address in the camera config settings as a result of being confused by erratic behavior of an AP or bridge. This can be mitigated (not necessarily corrected) by checking the 'Skip initial MAC, HTTP, DNS reachability tests' in the network IP configuration section of the camera's BI configuration.
No magic bullets. Just basic troubleshooting.