To allow CSF Firewall access to resellers, go to
WHM > Resellers > Edit Reseller Nameservers and Privileges
Under
Additional Software > Third Party Services
Select
ConfigServer Security & Firewall (Reseller UI)
Now you need to go to
WHM > Plugins > ConfigServer Security & Firewall > Edit Reseller Privs
Add
RESELLER_USERNAME:1:USE,UNBLOCK,GREP,ALLOW
For HostOnNet Reseller account, i added
hoston:1:USE,UNBLOCK,GREP,ALLOW