I installed a deco extender and some of my appletv apps don’t connect to the new hub.

What is your model Apple TV box? Identify your Apple TV model - Apple Support

deco extender

Is that related to mesh Wi-Fi?

YouTube and YouTubetv will not connect to my new hub …

I don’t think apps need to, nor can, connect to hubs. The Apple TV box needs to connect to the home network. Then all apps on that device can use that network connection. YouTube and YouTube TV don’t have hub interactions, afaik.

To configure the HomeKit setup, use the Home app on iPhone or iPad.

Add an accessory to the Home app - Apple Support.

Home app - Apple

Do you have an iPhone or iPad to configure HomeKit in the Home app? Try that.

The Apple TV box is just the passive hub, that helps accessories connect to HomeKit through iCloud, so that your other devices can access it and make the automations and scenes. The hub itself is not for managing or configuring.