Problem: need to flash service firmware

hello guyz. i need some help. I replaced the motherboard of my ZE551ml smartphone. The motherboard already has special service firmware, which is not for everyday using. So now i want to flash the firmware, to normal version. The problem is that i cant load into recovery mode, to apply ADB update. In bootloader mode, when i choose to reboot into recovery mode, phone is rebooting directly into the system. I tried to flash recovery (CN, WW and also custom TWRP versions ), flashing process is successful, but still cant load into recovery. is there any way to flash the firmware, without entering recovery mode? Thanks in advance.