Hello @Arjun 
No number can be ported to an existing mobile phone contract.
This is only possible when the mobile phone contract is commissioned.

(Translated with Google)

@Arjun 

The order of doing is following:

First let the old provider set an Opt-In or cancel your contract WITH announcement to port your number to a new provider

After confimation of this, order a new contract for your new provider also WITH anouncement to port a number into this contract.

Both providers agree to a due date when number is ported, old SIM is deactivated and new SIM is activated with the ported number.

New SIM must not be activated with printed number on it, that denies a porting of number afterwards.

Arjun

I ordered an annual plan from Telekom but I wanted to use my existing number from Lidl Connect.

Ordering was done online? When?

In the ordering process you can (actually: you must) tick the box that you want to port your number from an existing contract you have with a different provider.

So first of all you need to get rid of the unwanted Telekom contract - hopefully you are still within the 14 day period of "Widerrufsrecht".

Following that a new order of a Telekom mobile contract is required.

Lidl connect is prepaid - is your contract still active?