For an application to be reachable from the network, it must be allowed through the firewall. That terminology is a little backwards, but that’s how Windows calls it.
All reasonably recent versions of Windows maintain three firewall profiles for three types of networks: Public, Domain (not relevant for home users) and Private. This type is also what the Network and Sharing Center displays.
Windows uses various means to identify a network (as indicated by the network name). The only criterion relevant for home users is the default gateway’s MAC address.
When you switched ISPs, you received a new router. It has a different MAC address. Windows detected this and determined it is on a different network – technically correct.
New/unknown networks are Public by default. This is the most restricted profile and suitable for connecting to public networks like hotspots or your university or whatever.
When Windows asks you whether to allow a program through the firewall, the rule will only apply to the current network location(s) by default. That means you had rules for private networks. They simply did not apply after the router switch.
The best and most appropriate solution is to mark your network connection private again. In Windows 10, the corresponding switch is called “Make this PC discoverable”.
It is accessible in the Settings app → Network & Internet → Wi-Fi/Ethernet (depending on connection type) → Click on connected network.
Definitely not recommended, especially on laptops or tablets that frequently connect to foreign networks, is to modify firewall settings for public networks. This will leave your system vulnerable!