Hi
Wi-Fi calling sets up an IPSec VPN connection, this VPN connection in it's original form doesn't work across NAT, which is what we all have. So it has a work around method when it detects NAT which is to use port 500 and 4500. Port 500 is used just to setup the connection, then it is just port 4500. I suspect your router setting was a helper for IPSec VPNs to work normally across the NAT but the implementations are often buggy and unnecessary.
Turning that setting off doesn't affect security for your Wi-Fi calling. Your phone creates an encrypted VPN connection to the mobile phone companies servers, it can't create any other kind of the connection where Wi-Fi calling will still work, so no need to worry.
Regards
Phil