I think @Liquid77 that you might continue your research with your carrier on the basis that this might be an APN settings issue.
When I search the internet for “congstar + wifi calling + APN” I got many hits.
Another hint to this idea is that you just Upgraded Q → R; in /e/OS -r- branch, prebuilt/common/etc/apns-conf.xml · v1-r · e / os / android_vendor_lineage · GitLab there is no match for “congstar”, while in the Q branch there are two hits.
<apn carrier="congstar Internet" mcc="262" mnc="01" apn="internet.telekom" type="default,supl" protocol="IPV4V6" roaming_protocol="IP" authtype="1" user="congstar" password="cs" mvno_type="spn" mvno_match_data="congstar" bearer_bitmask="1|2|3|9|10|11|14|15" />
<apn carrier="congstar Internet" mcc="262" mnc="01" apn="internet.telekom" type="default,supl,mms" protocol="IPV4V6" roaming_protocol="IP" authtype="1" user="congstar" password="cs" mvno_type="spn" mvno_match_data="congstar.de" mmsc="http://mms.t-mobile.de/servlets/mms" mmsproxy="109.237.176.193" mmsport="8008" bearer_bitmask="1|2|3|9|10|11|14|15" />
This post does not give conclusive advice, nor does it mention wifi-Calling, but does demonstrate some of the issues found with #apn + #fp3 FP3 roaming/APN not working.
Of course they cannot see the APN settings on your device, and they may think that their settings have been issued to you on the fly. I suggest you call them again and ask to have the full APN settings sent direct to your device.
Less recommended
You can check your settings and try to edit them manually from Congstar’s own published APN settings. In my searches I found unusual variance in recommended settings as in this article (which I do not particularly recommend, but include only because it seems to show up unusual issues with Congstar’s settings).
https://www.internetsettings.org/en/germany/congstar-apn-settings/#apn-settings-for-android
Settings > Network and internet > Mobile network > Advanced > Access point names