the last time i tried this was way back in 2014 with win7 and bb-q10 and it worked through the bb proprietary software … but the phone became really hot even only on 3g … that’s 1.5 ghz dual core with 2 gb RAM; all mobile components … you know smartphone and all that jazz …
i’m looking forward to trying this with an L5 but i’m not holding my breath for a trully great experience on the v1 … even if it’s evergreen 
yes there are portable 3G/4G routers that can take a dedicated cellular SIM card or work with an external dongle through a usb port and have much better hw set-up in regards to heat management … BUT they are not open-hardware … some can have support for https://librecmc.org/
i see no RYF certified 3G/4G routers here YET > https://ryf.fsf.org/
don’t forget that even if you get a L5 from Purism the firmware of the modem is locked tight … and maybe this is true for all 3G/4G routers on the market as well (both open-hw or closed-hw)
the L5 is special in the fact that it’s manufacturer thought of ways to meet the risks of that closed-modem-firmware with some special EXTRA components on the PCB … search in the documentation … it’s been covered and here on the forums if i remember correctly > Who asked for the L5 schematics?