Your mobile phone can't transmit the repeater.

[Odessa Dawn]

"The strait shimmers as I board the Stella Australis. A sturdy yet elegant lady in blue and white, she is moored amid the flat warehouses of the docks, ready for another foray into the wilderness. We slip away as the sun descends. Within an hour, the signal on my mobile phone flickers and dies. Away to the west, Cape Froward – the southern tip of mainland South America – skulks invisible in the twilight, as we turn east in search of adventure."
More: Journey to the end of the Earth

When you are in a dead zone where your mobile phone can't transmit the repeater: How do we say it in informal English?
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[emsr2d2]

"The strait shimmers as I board the Stella Australis. A sturdy yet elegant lady in blue and white, she is moored amid the flat warehouses of the docks, ready for another foray into the wilderness. We slip away as the sun descends. Within an hour, the signal on my mobile phone flickers and dies. Away to the west, Cape Froward – the southern tip of mainland South America – skulks invisible in the twilight, as we turn east in search of adventure."
More: Journey to the end of the Earth

When you are in a dead zone where your mobile phone can't transmit the repeater: How do we say it in informal English?
​

I'm not sure what you mean by "your mobile phone can't transmit the repeater". However, you used the best phrase in your own post. You are in a dead zone, or a no-signal zone.

 

[Odessa Dawn]

I'm not sure what you mean by "your mobile phone can't transmit the repeater". However, you used the best phrase in your own post. You are in a dead zone, or a no-signal zone.

Dead zones
"Areas where cell phones cannot transmit to a nearby cell site, base station, or repeater are known as dead zones."
More: Mobile phone signal - Wikipedia, the free encyclopedia

 

[5jj]

"Areas where ceel phones cannot transmit to a nearby [...] repeater are known as dead zones."

That's fine, but in your original post you omitted a vital 'to', which was why ems did not understand what you meant. I didn't, either.