Gst under rcm

Hello,

I have purchased a web hosting, I provided GSTN to company, they didn't charged me GST in their bill, my GST mentioned on Invoice, company GSTN(other country GST/showing invalid) also mentioned. I checked lots in Gstr2a but not populated later, I have done this transaction on 12 Oct 2018, all returns filed regularly, but now I got to know it ahosho be paid under RCM, kindly guide, how can pay tax under RCM of Oct now.