Ein Netzbetreiber ist günstig, wohingegen der andere kostenloses Roaming anbietet.

This is a very common pattern in German when talking about two items in contrast.

• ein Netzbetreiber = one network operator / one provider
• der andere = the other one

The first noun is introduced as indefinite with ein. Then the second one is identified as the remaining member of the pair, so German uses der andere.

You could think of it as:

• One provider is inexpensive, whereas the other offers free roaming.

So der andere is definite because it refers to a specific other provider already understood from the context.

Yes. German often leaves out a noun when it is obvious from context.

So:

• der andere
  means
• the other one / the other provider

The full version would be:

• ..., wohingegen der andere Netzbetreiber kostenloses Roaming anbietet.

But repeating Netzbetreiber is not necessary, because it is already clear.

Grammatically, this is called using an adjective substantively or as a pronoun-like form.

wohingegen is a conjunction used to show contrast. A natural English equivalent is:

• whereas
• while, by contrast
• on the other hand

It is a bit more formal and more explicit than aber.

Compare:

• Ein Netzbetreiber ist günstig, aber der andere bietet kostenloses Roaming an.
  = simple contrast, more everyday

• Ein Netzbetreiber ist günstig, wohingegen der andere kostenloses Roaming anbietet.
  = clearer, slightly more formal whereas/by contrast

So wohingegen is especially useful when comparing two parallel facts.

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

Main clause:

• Ein Netzbetreiber ist günstig.

Subordinate clause:

• wohingegen der andere kostenloses Roaming anbietet

That final anbietet is exactly what you should expect after a subordinating conjunction like wohingegen.

Compare:

• Main clause: Der andere bietet kostenloses Roaming an.
• After wohingegen: ..., wohingegen der andere kostenloses Roaming anbietet.

Notice also that anbieten is a separable verb in main clauses:

• Er bietet kostenloses Roaming an.

But in subordinate clauses, the prefix stays attached:

• ..., weil er kostenloses Roaming anbietet.
• ..., wohingegen er kostenloses Roaming anbietet.

Because in German, subordinate clauses are separated by commas.

Since wohingegen introduces a subordinate clause, the comma is required:

• Ein Netzbetreiber ist günstig, wohingegen der andere kostenloses Roaming anbietet.

This is standard German punctuation.

Because Roaming is a neuter noun in German: das Roaming.

Here, Roaming is the direct object of anbietet, and there is no article in front of it. When an adjective comes before a noun without an article, it takes a strong ending.

So:

• kostenloses Roaming

Why -es?

• Roaming = neuter singular
• object of anbieten = accusative
• neuter singular accusative with no article takes -es

So the form is correct:

• kostenloses Roaming

Compare:

• das kostenlose Roaming
  with article
• kostenloses Roaming
  without article

Yes. It is an English loanword, but in German it behaves like a normal noun.

That means:

• it is capitalized: Roaming
• it has a grammatical gender: usually das Roaming
• it can take adjective endings and cases like other German nouns

So in this sentence, Roaming is not just an unchanged English word dropped into German; it is functioning as a proper German noun.

Because günstig is a predicate adjective here, not an adjective directly before a noun.

After verbs like sein, werden, and bleiben, adjectives usually stay in their basic form:

• Der Tarif ist günstig.
• Das Angebot ist attraktiv.
• Die Preise sind hoch.

So:

• Ein Netzbetreiber ist günstig.

But if the adjective comes before a noun, then it does take an ending:

• ein günstiger Netzbetreiber

So compare:

• Der Netzbetreiber ist günstig.
  predicate adjective, no ending

• Der günstige Netzbetreiber ...
  attributive adjective, with ending

It is nominative, because it is the subject of anbietet.

In the clause:

• der andere kostenloses Roaming anbietet

the subject is der andere, and the object is kostenloses Roaming.

If you expanded it to the full noun phrase, it would be:

• der andere Netzbetreiber

That is also nominative masculine singular.

Not exactly.

günstig usually means:

• inexpensive
• good value
• favorable

It often sounds neutral or positive.

billig can also mean cheap, but it sometimes suggests:

• lower quality
• something tacky
• something undesirably cheap

So in many contexts, günstig is the safer and more natural choice when you just mean a good price.

In this sentence, günstig suggests that one provider is attractively priced, not necessarily low-quality.

Yes, in many contexts it could.

For example:

• Ein Netzbetreiber ist günstig, während der andere kostenloses Roaming anbietet.

This is also correct and natural.

But there is a nuance:

• während can mean while in a time sense, or whereas in a contrastive sense
• wohingegen is more clearly contrastive and often sounds a bit more formal

So if you want a strong whereas/by contrast feeling, wohingegen is a very good choice.