I’d start with the simple stuff first.
If it calibrates on the ground but tilts down after takeoff, I’d check whether there is a gimbal mode or camera angle setting in the app, then try recalibrating while the drone is sitting completely flat and still.
Also make sure nothing is pulling on the camera or blocking the gimbal once the drone powers up. If it keeps doing it after calibration and firmware/app checks, I’d probably contact Ruko support instead of forcing the gimbal.