Purism is still actively conducting modem compatibility tests, including several 5G options. There are a couple primary contenders (@jonathon.hall may be able to elaborate after returning from travel).
Some modem options may not require substantial mechanical design changes, which is an important consideration because the injection mold process is extremely expensive (often >$100k). Other 5G modem options have proved functional but are physically larger and require modification of the internal frame and removal of the rear-facing camera. One minor known issue is that most modem alternatives have antenna connectors in different locations, which will make the engravings on the internal frame incorrect; it is inadvisable but possible to shave the injection mold to remove these markings for future batches.
Electrical requirements are being considered as well, of course, as the WWAN modem is directly powered by the battery, hence why the battery should never be charged in excess of 4.2V.