How can I disable my iPad's screen turning on when there is a notification?
I think, you have two (or maybe three) possibilities:

1. The "Do not disturb"-mode

When the "Do not disturb"-mode is enabled, your iPad won't make any sound and does not turn on the screen, when you get a notification. But you can still see the notifications on your lockscreen, when you turn on the display manually.



2. Turn of notifications for Telegram

If you turn of the notifications for Telegram, you won't be notified about any message. That would solve your problem, but I think you still want to see notifications for some of your chats?

(3. Mute some chats)

Since I'm not using Telegram, I'm not sure, if this is possible. But any modern messenger has options to mute individual chats for a while, so they won't send you push-notifications anymore.