信号不好。

You can say it as a standalone sentence. If you are on a call, just shout it out. 喂？信号不好！ (Wèi? Xìnhào bù hǎo!). You can also use it in text messages. Add a 对不起 (duìbùqǐ) at the start to be polite. It works as both a statement of fact and an apology. No complex grammar is needed here. Just subject plus the adjective phrase.

Use it when you are in an elevator. Use it when you are hiking in the mountains. It is perfect for professional video meetings. If your boss's face turns into pixels, say this. Use it when you want to end a boring phone call. It is the most polite way to 'hang up' accidentally. Everyone has experienced bad reception, so it is very relatable.

Do not use it if the connection is actually perfect. People can usually tell if you are lying. Avoid using it if the problem is your battery. For that, say 没电了 (méi diàn le). Do not use it to describe a person's personality. It only refers to electronic signals like Wi-Fi or cellular data. Using it for 'bad vibes' between people sounds very strange.

China has the world's largest mobile internet population. High-speed rail and skyscrapers are everywhere. Sometimes, these big buildings block the signal. In China, 'face' is important. Saying the signal is bad is a face-saving move. It blames the technology, not the person. It avoids the awkwardness of saying 'I don't want to talk.' It is a modern social lubricant.

You might hear 信号太差了 (xìnhào tài chà le) for 'signal is too poor.' If there is zero signal, say 没信号 (méi xìnhào). Young people might just say 卡了 (kǎ le), meaning 'it's lagged' or 'stuck.' If you are specifically talking about Wi-Fi, you can say 网络不好 (wǎngluò bù hǎo). All of these carry the same frustrated energy.