This official guide should help: Librem 5 Evergreen Help - #8 by Quarnero. As well, this current thought/idea of mine should work to get constant red LED light on:
Leave your Librem 5 for several hours (or overnight) like this and while still connected with your laptop or PC with USB-A to USB-C cable press once (or even twice) power button for a few seconds (not sure now if 18 seconds needed) to check if current battery already able to get your phone on. This prolonged charging of your Librem 5 battery, with around 0.5A, should initiate your completely drained mobile phone battery (all the way to zero, while phone turned automatically off at 2%, left alone for X days) to start charging normally (when connected to its original external power supply, up to 1.5A). Go back to the above official steps if needed.