subdomain not working with RUM

I have domain x.com

and I have subdomain   sub1.x.com

x.com will work great with RUM  >>is hosted by 3rd party 

sub1.x.com I cannot get to function >> is on prem (will only support https)

https://thwack.solarwinds.com/product-forums/pingdom/i/feature-requests

says this is available for subdomains? but this documentation seems very old.

Can anyone provide some help?