Hi @andi welcome to the /e/ forum.
If you were tying this to the SIM card provider (also partly indicated if WiFi connections are working fine) then the most likely setting that might change is the APN.
/e/ uses a list of APNs which is kept updated but some report that fully Googled phones get a better service of dynamic updating. You see this in some of the posts tagged #apn
You should be able to collect the published APNs of your carrier online.
It would be tedious to collect all the APN data by hand off Q and compare it with installed R. [1]
Thinking of the path indicated in this post No mobile data connection on Teracube2e, Windtre, Italy, we are introduced to apns-conf.xml.
You find it at prebuilt/common/etc/apns-conf.xml · v1-q · e / os / android_vendor_lineage · GitLab. I have linked v1-q. You can change branch to v1-r to see if there was an inappropriate change there.
Edit
[1] In the event you think the APN may be wrong the easiest thing is to ask the carrier to send the APN OTA, usually an automatic change happens in a few hours.
But going a bit deeper …
I did not find on my device apns-conf.xml in /etc or /system/ like this
$ ./adb devices
$ ./adb root
restarting adbd as root
$ ./adb shell
a3xeltexx:/ # cd /etc/
a3xeltexx:/etc # ls
a3xeltexx:/ # cd /system/
a3xeltexx:/system # ls
a3xeltexx:/ # find / -type f -name "apns-conf.xml"
find: apns-conf.xml: No such file or directory
So I extracted the whole list like this
a3xeltexx:/ # content query --uri content://telephony/carriers
Row: 0 _id=1, name=Test Internet, numeric=00101, mcc=001, mnc=01, carrier_id=-1, apn=VZWINTERNET, user=, server=, password=, proxy=, port=, mmsproxy=, mmsport=, mmsc=, authtype=-1, type=default,dun,supl, current=NULL, protocol=IPV4V6, roaming_protocol=IPV4V6, carrier_enabled=1, bearer=0, bearer_bitmask=0, network_type_bitmask=0, mvno_type=, mvno_match_data=, sub_id=-1, profile_id=0, modem_cognitive=0, max_conns=0, wait_time=0, max_conns_time=0, mtu=0, edited=0, user_visible=1, user_editable=1, owned_by=1, apn_set_id=0, skip_464xlat=-1
Row: 1 _id=2, name=Test FOTA, numeric=00101, mcc=001, mnc=01, carrier_id=-1, apn=VZWADMIN, user=, server=, password=, proxy=, port=, mmsproxy=, mmsport=, mmsc=, authtype=-1, type=fota, current=NULL, protocol=IPV4V6, roaming_protocol=IPV4V6, carrier_enabled=1, bearer=0, bearer_bitmask=0, network_type_bitmask=0, mvno_type=, mvno_match_data=, sub_id=-1, profile_id=0, modem_cognitive=0, max_conns=0, wait_time=0, max_conns_time=0, mtu=0, edited=0, user_visible=1, user_editable=1, owned_by=1, apn_set_id=0, skip_464xlat=-1
Row: 2 _id=3, name=Test IMS, numeric=00101, mcc=001, mnc=01, carrier_id=-1, apn=VZWIMS, user=, server=, password=, proxy=, port=, mmsproxy=, mmsport=, mmsc=, authtype=-1, type=ims, current=NULL, protocol=IPV4V6, roaming_protocol=IPV4V6, carrier_enabled=1, bearer=0, bearer_bitmask=0, network_type_bitmask=0, mvno_type=, mvno_match_data=, sub_id=-1, profile_id=0, modem_cognitive=0, max_conns=0, wait_
[...snip ...]
Row: 3471 _id=3472, name=Claro MMS, numeric=74810, mcc=748, mnc=10, carrier_id=-1, apn=mms.ctimovil.com.uy, user=, server=, password=, proxy=, port=, mmsproxy=, mmsport=, mmsc=http://mms.ctimovil.com.uy, authtype=-1, type=mms, current=NULL, protocol=IP, roaming_protocol=IP, carrier_enabled=1, bearer=0, bearer_bitmask=0, network_type_bitmask=0, mvno_type=, mvno_match_data=, sub_id=-1, profile_id=0, modem_cognitive=0, max_conns=0, wait_time=0, max_conns_time=0, mtu=0, edited=0, user_visible=1, user_editable=1, owned_by=1, apn_set_id=0, skip_464xlat=-1
Row: 3472 _id=3473, name=Claro UY, numeric=74810, mcc=748, mnc=10, carrier_id=-1, apn=gprs.claro.com.uy, user=ctigpr, server=, password=ctigpr999, proxy=, port=, mmsproxy=, mmsport=, mmsc=, authtype=-1, type=default,supl, current=NULL, protocol=IP, roaming_protocol=IP, carrier_enabled=1, bearer=0, bearer_bitmask=0, network_type_bitmask=0, mvno_type=, mvno_match_data=, sub_id=-1, profile_id=0, modem_cognitive=0, max_conns=0, wait_time=0, max_conns_time=0, mtu=0, edited=0, user_visible=1, user_editable=1, owned_by=1, apn_set_id=0, skip_464xlat=-1
Row: 3473 _id=3474, name=Claro UY MMS, numeric=74810, mcc=748, mnc=10, carrier_id=-1, apn=mms.ctimovil.com.uy, user=ctimms, server=, password=ctimms999, proxy=, port=, mmsproxy=170.051.255.240, mmsport=9201, mmsc=http://mms.ctimovil.com.uy, authtype=-1, type=mms, current=NULL, protocol=IP, roaming_protocol=IP, carrier_enabled=1, bearer=0, bearer_bitmask=0, network_type_bitmask=0, mvno_type=, mvno_match_data=, sub_id=-1, profile_id=0, modem_cognitive=0, max_conns=0, wait_time=0, max_conns_time=0, mtu=0, edited=0, user_visible=1, user_editable=1, owned_by=1, apn_set_id=0, skip_464xlat=-1
a3xeltexx:/ #
(See also adb shell content --help – if needed)
Perhaps you restored your Q build from a backup, which might include some dynamic change in the past to your APN.
If for your carrier the line had changed from the v1-q/prebuilt/common/etc/apns-conf.xml then this would imply you might need that change in R.