Why can I only mirror and not AirPlay from iPad

Thanks for your response, sorry for being unclear.

Airplay used to allow me to cast content from my iPad via AppleTV to an old Panasonic TV but now only lets me mirror it to the TV (small picture that doesn’t fill the screen). Following Apple updates rolled out to more modern TVs, I can cast iPad content direct to an LG 4K TV but again not via a second Apple TV downstairs.

What I need help with is casting content via my my Apple TV’s like I have done in the past.