Have you gone back inside the phone since the repair to look at the connections and the hardware of the cameras? The cameras may not be well connected to the main board. I’ve seen this issue on iPhone 8s in the past that needed new camera sensors, but if it happened after a repair, something may have just been knocked out of place. Disconnect the camera sensors (they’re under a metal plate held by two Y screws), lightly clean the contacts with iso alcohol and a soft toothbrush if you wanna be safe, and then reconnect the cameras once the alcohol is dry, if you used it.
Don’t reseal the front display until you can confirm that everything works correctly.
If you already did this and still have this issue, inspect the cameras and their cables for damage. If you open the camera app and try portrait or time lapse, do you see anything? Does the front camera still work?
I recent had the battery in my iPhone 7 replaced and my rear camera was only showing black screen, but I could take a selfie. I waited one day later, reset camera settings and powered down, held power button for 10-15 seconds, beat phone lightly on countertop, powered back on, camera started back working. Can take photos again☺☺I'm a happy camper