So you booted back into your USB installer, yes, went to the post install patch dialogue, yes, checked Recovery HD option, yes, clicked on Reboot, yes. Mac opened up in High Sierra, yes. Restart the Mac and hold the option/ alt key down select Recovery HD and press return, does the recovery drive start up or do you get the 'no file system' graphic.
If that is the case you could try re installing High Sierra again. Or if you can mount the Recovery HD on your Desktop you could alter the platform support list.
Open Terminal
enter diskutil list
from the list you will see on the right under IDENTIFIER the name of your Recovery HD, mine is disk0s3, yours may be different.
enter diskutil mount <IDENTIFIER>
you should get a confirmation and the disk should now be mounted on your Desktop.
Open the Recovery HD, click on com.recovery.boot
Open the PlatformSupport.plist in BBEdit or Text Wrangler.
Now replace one of the Board ID's with your Macs Board ID
Scroll down to the Supported Models list and replace one of the Macs model to your Macs model number.
Save.
Close.
Restart the Mac holding down the option/ alt key then select the Recovery HD.
It should now mount.