DNS problems only on mobile connection

Gamekiller48

A few days ago my Google Pixel 6 Pro suddenly started causing problems:
When navigating to websites with my mobile connection I would receive the error message "ERR_NAME_NOT_RESOLVED".
This happens only via my mobile connection, WiFi works fine.

More details:

I'm currently in a foreign country using data roaming
the phone worked just fine until two or three days ago.
I cannot pinpoint if there was any specific change that caused the weird new behaviour

Some other weird observations/solution attempts from my side:

my private DNS settings seem to get ignored. I had it set to the mullvad DNS, and changed it to the cloud flare address one.one.one.one during troubleshooting. Net analyzer however tells me that the WiFi connection uses the DNS server IP 79.79.79.79, while my cell connection uses the DNS server IP 62.109.121.17
This seems to be the likeliest culprit, but I have no idea how to actually fix this.
I checked that my VPN was turned off/not accidentally killing the connection with a kill switch
I checked chrome://net-internals/?#dns in vanadium and pressed clear host cache. Using the domain lookup field returns "An error occurred while resolving "Reddit.com" (net::ERR_NAME_NOT_RESOLVED" even on WiFi connection, but connecting to the site while on WiFi does work.
I restarted my device, no changes
I installed all the latest updates, no changes
I reset my network preferences via settings > system > reset network preferences & restarted, no changes
I set the private DNS setting to off & automatic, no changes in both situations
I installed a second browser (Firefox) to confirm the problem also appears in that browser.

I've tried everything that I could think off, and now I'm all out of ideas. Any suggestions would be appreciated.

Gamekiller48

I also found some other people having similar issues:
https://discuss.grapheneos.org/d/13381-pixel-6-no-mobile-data-ireland

Gamekiller48

I just observed a different error message as well:
ERR_ADDRESS_UNREACHABLE

Gamekiller48

The issue "fixed" itself (as in I can now connect to sites, I haven't checked if it actually uses the correct DNS yet) once I returned from my vacation, so it is related to roaming.
My working theory: my device used my provider's DNS, which is located in a different country, and the timeout for getting answers from the DNS server might be low enough to cause issues.
Is there a way to modify the timeout settings?

Also I"m not sure I want to give this topic the fixed tag yet, since I still get the feeling that this problem shouldn't have happened if the correct DNS server was actually selected.

Gamekiller48

DNS-settings still are completely ignored. My device does not connect to the DNS server set in the settings, but some other DBs server which I can only assume to be the provider default.