vincentzzzz Hi,
To enable diag port in qualcomm devices, please root device at first. After device successfully root, go to service tab, and click Read to switch phones to Diag port.
If above step fails, then you can try download enginering firmware(WW_ZB551KL_12.1.0.2.16GPCBACS28_factory.zip) in support, and flash via fastboot mode. By flashing engineering firmware, device will auto root and diag port also enable by default, so you can proceed rewrite QCN.