To get a fix with a GNSS receiver you need to have a good antenna connection and the GNSS module must have downloaded an almanac and the ephemerides for the satellites it is tracking. To speed up the process you can upload the almanac and ephmerides to the module , this is know as AGPS.
gnss_share can handle the AGPS portion and it also will share data with geoclue which is used by most L5 navigation software for geolocation.
You can copy the almanac and ephemeris from a phone that has a 3D fix to one that doesn’t and then have gnss_share upload it to the GNSS module.
If you don’t use AGPS the almanac and ephemeris will automatically get downloaded from the satellites. This takes at least 12 minutes and must have a clear view of the sky, preferably a window sill or outside. Once the initial navigation messages ( almanac and ephemeris https://en.wikipedia.org/wiki/GPS_signals#Navigation_message ) is downloaded it gets saved to battery backed RAM. If you are using gnss_share it will also get saved to the filesystem.
The second part of this is how well the antenna is receiving the satellites. As others have stated you can directly dump the output of the GNSS module using cat /dev/gnss0. You need to find the GSV sentences as these show the number and strength of the satellites being received ( section 11.4.4 https://www.st.com/resource/en/user_manual/dm00398983-teseoliv3-gnss-module--software-manual-stmicroelectronics.pdf ).
Initially you need to see a single satellite with a SNR of > 20 for the navigation messages to be downloaded and it must be active for more than 12 minutes. You’ll slowly see more and more satellites in the GSV messages. Once the module starts tracking satellites you will see a GSA message with satellites in it and that shows the current satellites being used for your location solution. After that you should see a GGA message that indicates a fix.
If you never see a GSV message with a SNR above 20 or a GSA message tracking satellites then it could either be a hardware problem or not enough of a view of the sky problem. The view of the sky problem can be cause by tall trees , tall buildings, metal or concrete roofs and the eaiest way is to take the L5 out into a field or some area with a full view of the sky.
If all the above fails then it may be a hardware problem. A common issue is the ground on the GNSS antenna https://puri.sm/wp-content/uploads/2021/03/22_v2.png. This part is both delicate and also must be touching the frame of the phone. On visual inspection if it is not touching the frame it can be GENTLY bent to touch it, see the dis-assembly instructions here https://puri.sm/posts/disassemble-librem-5/
