What that is this that

"The radio, that most ubiquitous of consumer-electronic appliances, is about to enter a new age."

How does "that" function in the sentence?

Is it a relative pronoun "that" or a "demonstrative adjective" . If it is a relative pronoun, then, doesn't it need one more "is" following it ? That said :

The radio, that is most ubiquitous of consumer-electronic appliances, is about to enter a new age.

thanks for your help..