I had never a problem with GPS. Worked always quickly and accurately. GPS malfunction is uncommon. Trying to see what GPS Test (free on Google Play) shows is a good idea. You'll know if some signal is received or the phone is completely blind.
The compass is another matter. It uses both magnetic sensor and gyroscope. Gyroscope might need calibration. Having Nougat, one may use code *#87# to enter engineering menu and find the gyroscope calibration option at the bottom (take care not to move the phone while pressing calibration on-screen button). The magnetic sensor may be affected by magnetized screw (upper left side screw is close to the sensor) and by some high current conductor inside the phone that makes peaks and drops on readout. While the screw may be demagnetized or exchanged with another the internal wire cannot be dealt with. Waving in 8-shape didn't help much and even if so din't help for long. I found the compass useless by design.
If your seller won't wish to fix the issue with GPS (and you didn't buy Aliexpress 1-year replace warranty) you'll have to fix it on yourself (like finding a repair shop). Good luck!
Blackview support? Screwdriver is your support.