How does international phone plans work?

Don't judge, just answer politely and seriously.
I have a Samsung Galaxy smartphone. I'm currently a MetroPCS customer. I have the $60 plan, which allows me to have unlimited 4G data, national call and msj So far I love this plan.
A month ago, I decided to add the $5 Mexico Unlimited international service. They told me, the service was based on my base rate plan, which means unlimited data. However, they said they didn't warranty me the speed data was going to be fast. But I have my doubts though. I checked the coverage map and saw many areas where there is no coverage(fortunately for me, the city where I used to live is a partner with MetroPCS). My question, however, is does No Coverage means that I won't get any signal? Or, will I get extra fees if I'm in those areas in particular?

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