You might want to take a look at UPnP. You can install UPnP via Luci (System > Advanced Settings)
user: root
your gui password.
System > software > update list
Find: luci-app-upnp
Install it. Refresh your page. Navigate along the main tabs to Services > UPnP
Enable UPnP.
On my router I have made some MiniUPnP ACLs (at the bottom of the UPnP page)
On my UPnP I only allow my two consoles to allow access to UPnP ports.
For you, here is what should work.
All these settings should be checked / enabled
Start UPnP and NAT-PMP service
Enable UPnP functionality
Enable NAT-PMP functionality
Enable secure mode
Allow adding forwards only to requesting ip addresses
Enable IGDv1 mode
Delete the default allow example but leave the default deny ACL
Create your own ACL
Comment: your device or description
External ports: 27015-27050
Internal addresses: 192.168.8.216
Internal ports: 27015-27050
Save
Ensure that your custom ACL is above the default deny (drag it higher up - 3 lines button) apply / save
Also ensure that the internal address of your device is reserved inside your glinet gui otherwise a change in client IP will render the rule useless.