I am trying to figure out why it does not work on Serial(0).
Be sure to read my warning: don't print something that looks like an AT command to Serial for debugging. It will also be sent to the OBD device.
What I need to enable if I would like read number of seen satellites?
Look at this table. If you need speed and satellites, the RMC sentence would give you both. That is the only sentence you would need to enable and parse. Enable RMC (and set LAST_SENTENCE to RMC) in NMEAGPS_cfg and add SATELLITES to GPSfix_cfg.h.
All fields have a validity flag, so you can make sure the satellites field is valid with this:
if (valid.satellites)
This can be valid even if other fields are not valid. The RMC sentence may have a satellite count without a location or speed.