Yes , as per rule 18 of the CGST rules , it is necessary to Display GSTN number on the board . As per section 125 General penalty up to of Rs 25000/- CGST and SGST 25000/- can be levied.
Since you have already paid the penalty however if the non compliance had taken place between 1st December 2020 to 30 September 2021 then application can be made for waiver of penalty.