Same issue here, Sony xperia XZ2 Compact Dual SIM. My wife on same network with iPhone8 has 4to5 bars, me zero. And yes it is also on O2 (in Germany), which I don't think is using a different technology compared to the others, just the coverage / signal strength is kind of miserable outside of the city. I know that the iPhone has no dual sim, still the difference is huge. In the settings the LTE is set as preferred, but the rest are also there, so the phone should switch automatically to a lower standard in case there is no signal or if the signal is very poor on LTE. In my case (SIM1 having no signal) what "helped" is that I have deactivated (Settings/Network/DualSim) my SIM2 (Telekom), the phone then restarted by itself (as the default data connection was set to SIM2 and at any network settings change the phone restarts automatically), I suddenly had 4bars signal on SIM1 I then activated SIM2 again, set the default data connection back to SIM2 ...and since then I have 2to4 bars on O2 (SIM1) and 2to3 bars on Telekom (SIM2). Both SIMs have LTE as preferred connection type. I did this just 3 hours ago, when I was looking for a solution for the problem, that's how I ended up on this forum. I thought I share with you my experience, maybe it helps... I'm sure the situation can be improved by SW. I don't know how serious Sony is taking this issue, but I got this phone because it has dual sim, but if I anyway cannot be reached on one of the SIMs, then I might not consider the brand next time... Still I do love my phone despite this "small" issue. We however might get "divorced" if the situation will not improve. I will post again if the situation will get worst again.
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