O celular não vai se conectar ao wi-fi porque o sinal está fraco.

In Brazilian Portuguese, it is very common to use the definite article with nouns, even when English would not.

So o celular literally means the cellphone, but in many contexts it can simply mean the cellphone/your cellphone/the phone, depending on the situation.

Portuguese uses articles more often than English, so O celular não vai... sounds very natural.

This is a very common way to talk about the future in Brazilian Portuguese.

• vai = is going
• se conectar = to connect

So não vai se conectar literally means is not going to connect.

Portuguese does have a simple future form, such as não se conectará, but in everyday spoken Brazilian Portuguese, vai + infinitive is much more common and natural.

Conectar-se or se conectar means to connect.

The se is part of the verb here. In modern Brazilian Portuguese, se conectar is a very common way to say to connect oneself / to get connected, especially for devices, internet, apps, and networks.

Examples:

• O celular se conectou. = The phone connected.
• Não consigo me conectar. = I can’t connect.

In casual Brazilian Portuguese, people often place the pronoun before the infinitive after vai:

• vai se conectar

That is very natural.

Ao is a contraction of:

• a
  • o = ao

Here, conectar-se a means to connect to something.

So:

• conectar-se ao wi-fi = to connect to the Wi-Fi

This happens because wi-fi is being treated as a masculine noun here, so it takes o:

• a + o wi-fi = ao wi-fi

Similar examples:

• Vou ao mercado. = I’m going to the market.
• Ela respondeu ao e-mail. = She replied to the email.

In Portuguese, borrowed words usually get assigned a grammatical gender. Wi-fi is commonly treated as masculine in Brazilian Portuguese, so people often say:

• o wi-fi
• ao wi-fi
• o sinal do wi-fi

That is why the sentence uses ao wi-fi.

Yes, in informal Brazilian Portuguese, many people do say conectar no wi-fi or se conectar no wi-fi.

However, se conectar ao wi-fi is generally considered more standard or more formally correct, because the usual pattern is conectar-se a.

So:

• standard / more formal: se conectar ao wi-fi
• common in speech: se conectar no wi-fi

Both are heard, but the sentence you were given uses the more standard structure.

Here porque means because, so it is written as one word.

Portuguese has several similar-looking forms:

• porque = because
• por que = why / for what reason
• por quê = why at the end of a question
• o porquê = the reason

In this sentence, it introduces the reason:

• ...porque o sinal está fraco. = ...because the signal is weak.

Portuguese often uses estar for a temporary condition or current state.

• está fraco = is weak right now
• é fraco would sound more like a permanent or defining characteristic

Since the Wi-Fi signal is weak at the moment, está fraco is the natural choice.

This is a very common ser vs. estar distinction:

• A internet está lenta. = The internet is slow right now.
• Ele é lento. = He is slow as a characteristic.

Adjectives in Portuguese usually agree with the noun they describe.

Here, fraco describes o sinal.

• sinal is masculine
• so the adjective is masculine too: fraco

If the noun were feminine, the adjective would usually change:

• a conexão está fraca = the connection is weak

So:

• o sinal está fraco
• a internet está fraca can also be heard in some contexts, depending on what is being described

In this sentence, sinal means signal, specifically the Wi-Fi signal strength.

It is a very common word in technology contexts:

• O sinal está fraco. = The signal is weak.
• Estou sem sinal. = I have no signal.
• O sinal do celular caiu. = The phone signal dropped.

So here it refers to the strength of the wireless connection.

Yes. In Brazilian Portuguese, celular is the normal everyday word for cell phone / mobile phone.

Examples:

• Meu celular está sem bateria. = My phone is out of battery.
• Esqueci o celular em casa. = I forgot my phone at home.

If you are learning Brazilian Portuguese, celular is the word you will hear most often.

Yes, the word order is completely natural:

• O celular não vai se conectar ao wi-fi porque o sinal está fraco.

Portuguese, like English, often puts the reason after porque.

You could rearrange it for emphasis, for example:

• Porque o sinal está fraco, o celular não vai se conectar ao wi-fi.

That is grammatically correct, but it sounds more deliberate or explanatory. The original version is more natural in everyday speech.

In natural speech, a few things happen:

• não often sounds roughly like nowng with a nasal ending
• vai sounds like vai with an English-like v
• se is usually a light see or sih
• conectar has stress on the last syllable: co-nec-TAR

So the phrase flows something like:

• não vai se conectar

In fast speech, the words link smoothly together, but each word is still recognizable. The nasal sound in não is especially important for sounding natural.

Yes. Once the phone has already been mentioned, you can replace it with ele:

• O celular não vai se conectar ao wi-fi. Ele está com problema.

Here ele refers to o celular, which is masculine.

That is another reason it is useful to notice grammatical gender:

• o celular → ele
• a internet → ela