Vou reclamar da fatura de internet ao gerente amanhã.

Vou reclamar is the very common ir + infinitive future in Brazilian Portuguese. It means I’m going to complain and is often more natural in everyday speech than the simple future reclamarei.

• vou reclamar = very common in speech
• reclamarei = correct, but more formal or less common in conversation
• reclamo = I complain / I am complaining, depending on context, so it does not specifically give the future meaning here

So Vou reclamar da fatura de internet ao gerente amanhã is a very natural way to say that you plan to do this tomorrow.

Here, reclamar means to complain.

In Brazilian Portuguese, reclamar de is the usual structure for to complain about something:

• reclamar da fatura = to complain about the bill

Be careful: in some contexts, reclamar can also mean something closer to demand or claim, but in this sentence the meaning is clearly complain.

Because reclamar normally uses the preposition de when you say what you are complaining about, and a fatura has the definite article a.

So:

• de + a = da

That is why you get:

• reclamar da fatura = to complain about the bill

This is a contraction, and it is extremely common in Portuguese.

Other examples:

• do = de + o
• dos = de + os
• das = de + as

Because this part shows to whom the complaint is directed.

• a = to
• o gerente = the manager
• a + o = ao

So:

• ao gerente = to the manager

In this sentence:

• da fatura tells you what the complaint is about
• ao gerente tells you who the complaint is made to

In this sentence, reclamar da fatura ao gerente is a more precise structure meaning:

• complain about the bill
• to the manager

You may also hear people say things with para in everyday speech, especially in conversation, but reclamar de algo a alguém is a more traditional and structurally clear pattern.

So in learner terms:

• reclamar da fatura ao gerente = complain about the bill to the manager
• reclamar para o gerente may appear in spoken language, but it is less exact unless the rest of the sentence makes everything clear

Often, yes, in many contexts they are very close.

• fatura usually refers to an invoice or billing statement
• conta is often the everyday word for a bill

So:

• fatura de internet = internet invoice / internet bill
• conta de internet = internet bill

Depending on context, one may sound slightly more technical or more everyday, but both are understandable.

Because de internet works like of internet / for internet service in this kind of noun phrase.

In Portuguese, service types often appear without an article after de:

• fatura de internet
• conta de luz
• conta de água
• plano de saúde

So fatura de internet is a normal way to say internet bill.

Yes, amanhã can go in different positions.

The sentence:

• Vou reclamar da fatura de internet ao gerente amanhã.

is perfectly natural and means I’m going to complain to the manager about the internet bill tomorrow.

You could also say:

• Amanhã vou reclamar da fatura de internet ao gerente.
• Vou amanhã reclamar da fatura de internet ao gerente. (possible, but less neutral)

Putting amanhã at the end is very common and natural. Putting it at the beginning can add a bit more emphasis to tomorrow.

Normally, amanhã is understood as the time of the action vou reclamar: I will complain tomorrow.

It would not normally mean the bill for tomorrow. The structure strongly points to tomorrow modifying the whole action.

If someone wanted to talk about a bill connected with tomorrow, they would probably phrase it differently.

No, not in this sentence.

• gerente is a masculine noun here, so the article is o
• after the preposition a, a + o = ao

So you need:

• ao gerente

You generally would not say just a gerente here, because that would suggest a feminine noun phrase, and gerente in this sentence refers to the manager with the masculine article o.

Without the article, the sentence would sound incomplete or unnatural in standard usage.

Yes, it is natural and correct.

A Brazilian speaker might also say:

• Vou reclamar da conta de internet com o gerente amanhã.
• Amanhã vou reclamar da fatura de internet com o gerente.

But your original sentence is grammatically solid and clear:

• Vou reclamar da fatura de internet ao gerente amanhã.

If anything, some speakers may prefer com o gerente in casual speech, but ao gerente is absolutely valid and clearly expresses to the manager.

Here is the breakdown:

• Vou reclamar = verbal phrase, I’m going to complain
• da fatura de internet = what the complaint is about
  • da = de + a
  • fatura de internet = internet bill
• ao gerente = the person receiving the complaint
  • ao = a + o
• amanhã = time expression, tomorrow

So the structure is basically:

[future action] + [about what] + [to whom] + [when]

That makes it a very useful model sentence for building similar ones.