Hello,
This is the same project as described in this post with the same setup.
In the process of programming more boards to have the fast switching capability allowed by using NB-IoT, we encountered an issue where some boards that were running on LTE-M and then forced to run on NB-IoT will not be able to get a first fix for GPS for many hours due to insufficient time windows. The board will eventually get GPS, but it will take a long time again after a reset.
If we let the board run while connected to NB-IoT for about a week however, we notice that the issue goes away. Also, if the board starts off by connecting to NB-IoT, the issue doesn't occur. For boards that have this issue, if we turn disconnect cell the gps is able to get a first fix fairly quickly, and will continue to get fixes after cell is connected again.
For context, we are connecting to cell on startup, downloading AGPS data, and then using GPS on the 1HZ continuous mode while cell is connected. The LTE-M network here gives us PSM and eDRX parameters while the NB-IoT network here does not.
My guess is that there are conflicting DRX parameters that interfere with the modem radio giving time for the GPS to decode the satellite ephemerides it receives while being connected to cell, and that this is something that is set on the network side. Boards that we have been testing this specific implementation with already don't seem to have an issue switching between LTE-M and NB-IoT, just boards that we are switching from using both LTE-M and NB-IoT to only NB-IoT.
I hope that the Nordic team can help shed some light on this issue.
Thanks,
Stuart