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Not legally, since all of the frequencies in question require licensing and most are allocated to mobile phone providers.

But yes, this is already possible, in fact, Osmocom has a fully open baseband running on certain phone chipsets.

Note that the Osmocom baseband project isn't aimed at end-user devices, only at researchers. For example it runs part of the stack on the laptop the phone is attached to.

I guess this is GSM, but I heard 5G operates on some license free bands.

The CBRS band (48, 3.6Ghz) could be used for this. Support for this band only exists in the very newest smartphones.

Due to the frequency division/time division duplexing nature of 3GPP protocols, a mesh network is very challenging and non-trival. The 5G standardization has some study items on multi-hop networks, but that is a ways out and I’m not sure how and if it will even manifest in the standard.