This is the dreaded IMEI corruption issue.
For this specific model, technicians generally use specialized software suites: ChimeraTool : A popular professional option for repairing IMEI on MediaTek and Qualcomm devices oppo cph1937 imei repair
Repairing a null IMEI to its original factory state is generally a legitimate repair. However, This is the dreaded IMEI corruption issue
to rewrite the original IMEI back to the device's partition. Stock Firmware Flashing oppo cph1937 imei repair