Small routers exist. Take for example this battery powered GL-MiFi [1] LTE router. Comes embededded with OpenWRT so your LTE connection can utilize Wireguard, Shadowsocks, OpenVPN, etc. Would pair well with something like the PinePhone.
Hm. That looks cool as fuck. Do you happen to know if it needs to be specially provisioned with e.g. AT&T, or can one simply take the LTE sim card out of their phone and stick it into one of these?