For all searching for this Problem with Teams not showing the calender app using hybrid szenario.
Refering to this basic guide:
Sophos UTM: Create WAF to allow traffic through Exchange 2016
using the MS Connection Tool:
using a account with teams license (temporarily) active I ran the test and noticed that Entry URL /api/... was used.
So do 2 things:
Add "/api" as an Entry URL to the Exchange Webservice Firewall Profile
and add "/api/*" to the URL-Hardening exception.
Rerun the Test: all fine.
After 10-15 Minutes, customer restarts Teams correctly, the calender app is where it used to be.
This thread was automatically locked due to age.