Provide the topology pls. Which port did you open? What's the firmware version of your W9980 and ER6120?

linksys e2500(LAN)----(LAN)SG108(LAN)----PC? PC won't obtain IP address? If PC is connected to LAN port of the router, it works?

Can you provide a screenshot of login page? It should be admin for both. You may reset the powerline extender and try again.

If Share Selected Folders is selected, only the folders you specified will be shared. Youhave to click the Create Share Folder appeared to specify folders allowed to be shared. That's what it means,...

Did you try it via different web browsers?

If the router itself doesn't assign correct IP address, it won't help by connecting powerline devices. The router should be the DHCP server. If its DHCP is working abnormal, you should check it....