Q: What if it has been set up already on a different tv. How then do I connect to it with another iOS device as a remote?

are you trying to control two devices from the same iOS device as a remote?

are both devices in proximity to the controlling device.

see this document from Apple

Set up the Apple TV Remote app - Apple Support

Apple TV - Official Apple Support

You do need the real remote to configure the ATV network settings, and to allow the iOS Remote app to connect. Do you see the ATV desktop when connected to the TV?

Depending on which Apple TV you have you may be able to set up the Apple TV from your mobile device. You will need a 3rd generation Apple TV for later running software version 6.0 or later and an iOS device running iOS 9.1 or later.

Restore your Apple TV. When the Apple TV setup screen appears, unlock your iOS device and make sure Bluetooth and Wi-Fi are turned on. Touch your iOS device to Apple TV and follow the onscreen instructions on your iOS device and Apple TV.

Automatic setup doesn’t transfer account information for third-party apps such as Netflix or Hulu, and doesn’t turn on iTunes Match or Home Sharing.

If this doesn't work, you don't have an iOS mobile device or your equipment doesn't meet the minimum requirements, you may wish to look at ciderTV for Mac OSX, once you have installed the app on your Mac...

• Check that Bluetooth is On & Discoverable;
• Keep Bluetooth preferences window open during pairing;
• Connect Apple TV to your Mac via USB cable and reset it in iTunes app;
• Turn Apple TV on and wait until it finds your Mac. A pop-up window will appear that says “Pairing Request from Apple TV”;
• Tap "Pair";
• You will see CiderTV control interface with buttons.