As the points out, Android 12 adds a toggle to disable the 2G modem. 2G is insecure and vulnerable to interception, so you're advised to avoid it.
eff.org/deeplinks/2022
Unfortunately, this feature requires version 1.6 of the Radio HAL, and many devices don't have that.
Conversation
GrapheneOS has supported LTE only mode for years. It's something we consider an attack surface reduction feature. Avoiding 2G/3G doesn't make cellular networks secure/trustworthy. It's important to use authenticated encryption and avoid trusting networks.
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LTE only mode is good, but extremely rarely my phone switches to HSPA when there is bad reception.
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That means you aren't using LTE only mode. LTE mode is not LTE only mode. You can verify you're using LTE only mode by disabling VoLTE and testing a call. If you can make a call, you aren't using LTE only mode.
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I do know that I’m not using LTE only mode, I just said that that’s a problem if you’re using LTE only mode.
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Ah, that makes sense.
It's worth noting that the phone can often successfully keep using LTE with low quality reception instead of falling back to 2G/3G. LTE and below mode isn't very aggressive at sticking to LTE.