Oppo A5 2020 (CPH1931) won't go in to fastboot mode/bootloader

I want to root my phone, install custom recovery and eventually a custom ROM. For this, I need to unlock the bootloader. I have an Oppo A5 2020 CPH1931. It should go into bootloader when I press volume+ and power buttons together for 5 seconds after completely turning off the phone. But when I do this, it boots like normal. Tried all different combinations and durations. I am able to boot into recovery but not fastboot mode. I have turned on the "OEM unlocking" option in developer options. I tried it through ADB, it says fastboot on the screen for half a second and then once again jumps back to normal booting. I also tried to already put the fastboot command "fastboot oem unlock". When it says I use another terminal to try to boot into fastboot with ADB. But this does not work (also tried this with "fastboot flashing unlock"). By the way, my devices only shows up with adb devices when I select transfer files upon connecting the USB cable. When I select just charge it doesn't even show. There are tons of guides online on how to enter the bootloader of this device, so it must have been possible at some point. I considered trying to revert to an old STOCK ROM that still has fastboot. I contacted Oppo about this but they keep trying to confuse me and then just left the conversation. **Is there a way to go back to an old STOCK ROM that still supports fastboot mode?** **EDIT:** I tried using the EDL toolbox provider by Björn Kerler like Alecxs suggested, but it cannot find a loader for my device: Qualcomm Sahara / Firehose Client V3.53 (c) B.Kerler 2018-2021. main - Trying with no loader given ... main - Waiting for the device ...... main - Hint: Press and hold vol up+dwn, connect usb. For some, only use vol up. main - Xiaomi: Press and hold vol dwn + pwr, in fastboot mode connect usb. Run "./fastpwn oem edl". main - Other: Run "adb reboot edl". ............... ...main - Device detected :) main - Mode detected: sahara Device is in EDL mode .. continuing. sahara - ------------------------ HWID: 0x0010a0e100510000 (MSM_ID:0x0010a0e1,OEM_ID:0x0051,MODEL_ID:0x0000) CPU detected: "nicobar" PK_HASH: 0x6e91b529aad0307f1fc67310dab1ff821eb7aa02fa5620157a9d20e124f4e0b8 Serial: 0x31276e91 sahara sahara - [LIB]: Couldn't find a loader for given hwid and pkhash (0010a0e100510000_6e91b529aad0307f_[FHPRG/ENPRG].bin) :( Device is in an unknown sahara state, resetting resp={'cmd': 1, 'len': 48, 'version': 2, 'version_min': 1, 'max_cmd_len': 1024, 'mode': 0, 'res1': 0, 'res2': 0, 'res3': 0, 'res4': 0, 'res5': 0, 'res6': 0, 'object_size': 48, 'raw_data': bytearray(b'\x01\x00\x00\x000\x00\x00\x00\x02\x00\x00\x00\x01\x00\x00\x00\x00\x04\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')}