Why can I only turn on iMessage when Wi-Fi is on (iPhone)?

I have an iPhone 5s, and when I'm out of the house I turn iMessage off to send photos so it doesn't cost me data. When I try to turn it back on (Cellular, no Wi-Fi) it says I have to turn on Cellular or Wi-Fi (Cellular is already on) how can I fix this?

Settings> General> Cellular> turn off

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