CC doesn't connect to an iPad. Instead, CC on the iPad connects to the computer running calibre. I assume that is what you meant.
My bet is that the new modem (home wireless router) is enforcing one of
- Guest mode. This is where one or more of the wireless networks (SSIDs) are prevented from connecting to cabled devices or devices on other wireless networks.
- Wireless Isolation: this means that a wireless device cannot see any other device on the network, cabled or otherwise.
- Band blocking: preventing devices from communicating with other devices on a different wireless band (2.4GHz vs 5GHz).
It also could be that the computer running calibre is seeing a new network topology and is blocking connections. On Windows this would probably be evidenced as the "network" not being "home".