Rcm under gst

Hi Friends

I had a few queries wrt GST. Hope you would help me out with the same.

If the supplier has applied for GST Registration and is awaiting the same, should the buyer pay GST under RCM.

Also if the buyer fails to pay GST under RCM for any previous month as a result of omission, can he make the payment in the subsequent month through GSTR-3B?

Thanks in advance