Hi,
Since summer / fall (not entirely sure) I have been having issues when traveling, basically having unusable mobile internet. Fortunately I found a workaround.
I use a pixel 6a with uptodate grapheneos. My physical sim card is from a local carrier and I usually use it for mobile internet / calls etc at home. I use e-sims when traveling.
When traveling, I had unusable mobile data. Very unstable connection, sometimes network would show up unavailable (with !) also on my main sim. What I noticed was that the netork indicators were flashing, as if the mobile network was being reset every few seconds.
I initially blamed my Esim carrier as I was using services like airalo, and being highly frustrated to not being able to have stable calls on WhatsApp even?on just audio. Browsing would work most of the time, but sometimes with loading delays.
A speed test on a 5G connection from a main carrier e-sim was giving me fluctuating results, going from 800kbps to 11MBPS to 3 MBPS (vs 500 Mbit/s on 5G that I can get normally). I tried setting a fixed network, changing the network to LTE only or 4g, to no avail. Of course when switching esims I never chose the option to use the 2nd mobile connection as a backup since it does not make sense in my scenario
While trying to find the issue I disabled the physical sim and found that my network was much more reliable this way, no more flashing of the network and better / consistent speeds. However I like to receive calls and SMS, so this wasn't cutting it. While checking further I realized that if I switched my physical SIM to 3G (even though mobile data is off for that sim!!) when using the e-sim the issue is gone. It also works if I put my physical sim to LTE only, the issue is when I leave it at LTE recommended. The weird thing is mobile data is off for that sim, I never choose the option to use the mobile network as backup.
I will try to capture adb logs since I suspect that the flashing of the network icons should appear there. Not sure whether this is a phone issue or software or grapheneos.
Its a huge relief for me, so hopefully this helps someone else.