Hi - not sure, but this may be an issue with the HDMI cable and content you are attempting to watch. I think if you have an Apple TV3 or Apple TV4 - HDMI 1.3 or 1.4 cable to Full HD TV should work OK - I have both devices and did not have to upgrade my cables - my TVs are 10 years old.
If Apple TV4 and TV is a 4K UHD HDR type - I think you will need a HDMI 2.0 cable to see 4K content as these cables carry a form of copyright protection signalling - you indicated you could hear the audio. If your cable is compatible and you are still having issues, maybe a Reset of the Apple TV to see if improvement occurs.
Apple iPad Screen Mirroring always has some issues for me - I found it troublesome at times and it didn't allow consistent experience in controlling programs SBS On Demand and ABC iView particularly when the program finished - it was one of the reasons I bought the Apple TV4 which has a superior customer experience to the Apple TV3 in that and other areas - e.g. voice search and control. The Apple TV4 has native SBS On-Demand and ABC iView Apps which are not on the TV3; since upgrading to the TV4, I have no need to use Apple Screen Mirroring anymore.