These instructions assume "example.com" is the domain for your NextCloud. Adjust according to your setup.
Add BotFather
/newbot
nameof_bot
Save the API key somewhere you will need to put it in nextcloud along with your bot username in the social login section.
Set domain of you nextcloud instance
/setdomain
example.com
Go to your nextcloud settings social login admin page.
Enter your Telegram bot name in App id field
nameof_bot
and your API key that you copied when you created your bot to Secret field
And thats it
Telegram auth will not work everywhere another than browser.