Or does a service tech have to install?
The basic requirement is the outlet the portal is connected to has to be wired to the same leg of cable that the other Gateway and Portals are on. So when the original installer was there to setup your Gateway, if you had let him know you planned to add a Portal on that outlet he would have wired it so that you could just connect a Portal in the future and you would be good to go.
If that is not the case, we would need to send a tech to adjust the splitter configurations.
Generally, Unless the original install tech told you an outlet is ready for a portal install, you will need a tech to install a new portal - It uses special filters that are not otherwise on the lines.
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