Hello,
You should be receiving the RSRP and RSRQ values using the GET /devices/ or GET /devices/{id} endpoints. Also, the values fall under dynamic information so it should get updated. So it might be possible that your device signal strength values simply do not change in that time. Are you still having this issue and the signal strength values have not updated?
The API call response date and time can be found in the Headers of the response, more specifically - the Date value.
The time when the last information update in RMS for the device happened can be gathered in the same API call (the last_updated_at value, in UTC time).