Notice for Suo Moto Cancellation of registration of GST registration

Dear All

We have received a show cause notice Suo Moto for cancellation of registration of GST.

We are a small service provider and turnover is below 1.5 Cr so we choose the frequency of filing of GSTR-1 quarterly but in portal it is showing monthly. We think it is some mistake of GST and we continue filing GSTR-1 quarterly and 3B monthly without fail.

As the return already filed in 2019-20 system is not allowing to change the frequency .

Now department sent the show cause notice.

What to do ?

Pls advice 

Thanks & Regards

Gupta