There are a lot of questions that would need to be answered to fully help you.
But, I will assume some things.
I assume you are not in the US, so you datum and coordinate system could be WGS84 with ellipsoid , but I don't know.
If you flew RTK and had FIX it is pretty easy.
Open Pix4D
Name Project
Import Images, make sure you choose the correct datum for the images on this screen. For me it is NAD83(2011) Geographic, and use the Height Above ellipsoid for vertical system with the field being 0 (This means that the elevations are ellipsoid values) For you the datum will be whatever your DRTK was using (Setup on a point or receiving corrections from an RTN RTK NTRIP service).
Import your control points, choose the datum and coordinate system that they are in along with vertical and then use the Basic Editor to mark them in 5 to 10 images. Assign them as either a Check or GCP.
Choose Maps and Models for output
When you get to the project screen, you can still go in and change the output datum and coordinate system if they differ from your control points. You want to output in a projected system (Most Likely, but I am not sure about your goals).
Run Step 1 only to create sparse cloud.
After sparse cloud is completed, use the GCP editor to fully mark all control points and assign the control points as either GCP or Check Point.
Reoptimize
Ensure that everything looks good.
Run steps 2 and 3.
Your biggest problem will be the datum and coordinate system you used with the DRTK. If you are receiving RTN RTK corrections the service can tell you what datum they output in as well as vertical datum and that will be your imported images datums.
If you setup your DRTK over a known point, then whatever datum the point is, that is what datum your drone images are in.
If you just setup the DRTK by itself where it is not over a known point or receivng RTN RTK corrections it is in WGS84 ellipsoid and not very accurate even with averaging that point.