In this sentence, does "which " refer to "the mobile and internet infrastructure that existed"?

"Which" refers to what precedes it, i.e. "ease". Theoretically, it could refer to the entire phrase "the mobile and the internet infrastructure that existed and the ease". Having it refer to that would not be grammatically incorrect, but it would be unclear.

There are two clauses, both dependent, here: "that existed" and "which customers were able to access technology". This is actually a sentence fragment, not a sentence, because both of the verbs are in subordinate clauses. The entire phrase "the mobile and the internet infrastructure that existed" functions as a noun, as does "the ease with which customers were able to access technology". So this quote consists simply of two nouns connected with "and" and no main verb.

Nope. The sentence is talking about two separate things:

1. The mobile and the internet infrastructure that existed.
2. The ease with which customers were able to access technology.

None of the words in phrase 2 refer to anything in phrase 1.

In phrase 2, the word "which" refers back to "ease". Phrase 2 is saying that customers were able to access technology with ease, and the phrase refers to that ease.

There is only one clause here, and that is the dependent clause "with which customers were able to access technology".