Firewall is blocking iPad internet access

Hello,

my iPad is connected to the internet via WiFi and a DSL-Router/Modem with a build-in firewall. Unfortunately, some of the applications on the iPad have problems going through the firewall, e.g. Facetime, all of my configured EMail-Accounts or Find-My-iPad. If I turn off the firewall in the router everything works fine. Of course I don't want to to turn off the firewall. I assume the problem is, that I have to allow these services/ports to pass through the firewall. Is there any way to find out what ports these services are using, so that I can open them in the firewall?

Best regards

Alexander