Die Flatrate ist unbegrenzt, wohingegen die Datenautomatik zusätzliches Datenvolumen zubucht.

Yes. Flatrate is a loanword used in German, especially in telecom and internet contexts. In German, it is treated as a normal noun, so it gets a grammatical gender and article: die Flatrate.

A native English speaker might expect it to behave like English, but in German even borrowed words normally take an article.

So in this sentence:

• die Flatrate = the flat-rate plan / unlimited plan

It is feminine, which is something you simply have to learn with the noun.

Because unbegrenzt is not being used directly before a noun here. It is a predicate adjective after the verb sein.

Compare:

• Die Flatrate ist unbegrenzt. = The plan is unlimited.
• die unbegrenzte Flatrate = the unlimited plan

In German:

• attributive adjectives (before a noun) take endings
• predicate adjectives (after sein, werden, bleiben, etc.) do not take endings

So:

• ist unbegrenzt = correct
• ist unbegrenzte = incorrect

Wohingegen means whereas, while, or by contrast.

It sets up a contrast between two statements:

• Die Flatrate ist unbegrenzt
• wohingegen die Datenautomatik zusätzliches Datenvolumen zubucht

So the overall idea is:

• the flat-rate plan is unlimited,
• whereas/by contrast the automatic data feature adds extra data volume.

It is a fairly formal or written-style connector. In more everyday German, you might also see:

• während
• aber
• im Gegensatz dazu

But wohingegen is perfectly natural here.

Because wohingegen introduces a subordinate clause, and in German subordinate clauses send the conjugated verb to the end.

Main clause:

• Die Flatrate ist unbegrenzt.

Subordinate clause:

• wohingegen die Datenautomatik zusätzliches Datenvolumen zubucht

That is why zubucht comes last.

This is a very important German word-order pattern:

• main clause: verb usually in position 2
• subordinate clause: verb usually at the end

Datenautomatik is a compound noun:

• Daten = data
• Automatik = automatic system / automatic feature

Together, it refers to a mobile-phone feature that automatically adds more data once your normal data allowance is used up.

The gender comes from the final part of the compound:

• die Automatik is feminine
• therefore die Datenautomatik is also feminine

This is a general rule in German compounds: the last element determines the gender.

Yes. Zubuchen is a separable verb.

Basic idea:

• buchen = to book / add / reserve
• zubuchen = to add on additionally, especially to an account or plan

In telecom language, Datenvolumen zubuchen means to add extra data to your contract or account.

In a normal main clause, the prefix separates:

• Die Datenautomatik bucht zusätzliches Datenvolumen zu.

But in this sentence, the verb is inside a subordinate clause introduced by wohingegen, so the prefix stays attached:

• ..., wohingegen die Datenautomatik zusätzliches Datenvolumen zubucht.

So yes, it is separable, but separable verbs are not separated in infinitives, participles, or subordinate clauses.

Because zusätzliches is an adjective placed before the noun Datenvolumen, and it must agree with the noun.

Here:

• Datenvolumen is neuter: das Datenvolumen
• it is in the accusative singular here, because it is the direct object of zubucht
• there is no article before it

When there is no article, the adjective usually takes the ending that shows the grammatical information itself. For neuter singular nominative/accusative, that ending is -es.

So:

• zusätzliches Datenvolumen = additional data volume

You can compare:

• das zusätzliche Datenvolumen
• ein zusätzliches Datenvolumen
• zusätzliches Datenvolumen

Because in German, Datenvolumen is usually treated as a mass noun or abstract quantity, similar to data volume in English.

So here it does not mean individual data units, but rather an amount of mobile data.

That is why singular works naturally:

• zusätzliches Datenvolumen = extra data volume / additional data allowance

German often uses singular nouns for this kind of concept:

• Wasser
• Geld
• Internet
• Datenvolumen

Yes.

In the clause:

• die Datenautomatik zusätzliches Datenvolumen zubucht

the parts are:

• die Datenautomatik = subject
• zusätzliches Datenvolumen = direct object
• zubucht = verb

You can tell that die Datenautomatik is the thing performing the action of adding the extra data.

So literally, the clause says:

• whereas the automatic data feature adds extra data volume

Because German uses commas to separate a main clause from a subordinate clause.

Here you have:

• main clause: Die Flatrate ist unbegrenzt
• subordinate clause: wohingegen die Datenautomatik zusätzliches Datenvolumen zubucht

German punctuation is stricter than English in this area. When a subordinate clause begins, a comma is normally required.

So the comma here is not optional.

Yes, that would also be grammatically correct, but the nuance is a little different.

• aber = simple but
• wohingegen = whereas / by contrast

Your version with aber sounds more conversational and direct.
The original with wohingegen sounds a bit more formal and emphasizes contrast more clearly.

Also notice the word order difference:

With aber (main clause word order):

• ..., aber die Datenautomatik bucht zusätzliches Datenvolumen zu.

With wohingegen (subordinate clause word order):

• ..., wohingegen die Datenautomatik zusätzliches Datenvolumen zubucht.

Both are possible, but they are structured differently.