I would like to ask all of you for help regarding a strange problem with an Oppo smartphone.
A week ago, I bought for my son (11 years old) a second hand Oppo Reno 5G smartphone on an online marketplace here in the UK. He is more proactive than me with smartphones and he said that the phone can not be updated and that he can not download from Google Play reference apps like Antutu or 3D Mark. All other games or device information applications can be installed without any problem. The phone displays in the file manager 0 B an unused space but I can copy files, take pictures and install games without any problem.
The phone was supposed to be an Oppo Reno 5G and looks identical to Reno 10X. On the side of the body is written tiny letters "Model: CPH1919 Made in China" The verified online imei on a few sites returns Oppo CPH1921.
Specifications as shown in all device information applications: Snapdragon 845, Andreno 640, 8GB RAM, 256GB, 12Mp or 48Mp camera.
Model presented in the phone PAFM00. In the software update menu is written "This is already the latest version ColorOS 6 PAFM00EE_11_A.01".
I have studied this issue during the last 2 days during my free time. Oppo adb drivers installed and traded with adb android drivers. Here is the result.
With the phone turned off, when I press the Volume Down and Power On / Off buttons, it flashes an image for a quarter of a second with a blinking head icon with headphones and Chinese text that I'm using. I translated: "Entered Fastboot Press and hold the power button for 6 ~ 10 seconds or reinstall the battery and quit". As long as I hold down these buttons, it flashes every 3 seconds. If I release them, the phone restarts and the Oppo logo returns to normal.
Using the ADB tools (different version, Studio), only the "adb devices" return the model number and the "device" afterwards. All other adb commands (reboot adb, usb, reboot-bootloader and adb restart recovery, etc., have tried most of them) have returned "error: closed", for "fastboot devices" ", nothing is displayed," fastboot flashing unlock "i get" waiting for the device ".
I have tried to install the Rom stock. By using the volume and power up buttons with QPST, I sometimes get Qualcomm and sometimes "Q / QCP-XXX (Sahara Download)" because the phone restarts from a way or another just after releasing those buttons and lost the connection.
Any advice to update this device and have it displayed correctly the correct model will be great.

About the phone, quick start, specifications

Specifications 2, specifications 3, reference