iPhone 5c doesn't support 4G?

The 5C supports 4G (LTE) if your carrier’s network supports it. However, you will need a 4G SIM card from your carrier, and you may have to update your carrier settings—>Update your carrier settings on your iPhone or iPad - Apple Support

If you want 4g you will have to get a newer phone

If your phone was sold for use in a different region of the world it may not support the LTE bands being used in your country.

You don't.

A 5 c can't run anything higher than 10.3.4

Interesting; it must be a local restriction. There’s nothing you can do about it, I’m afraid.

You can not upgrade that device to iOS 11.

No. You can not.

No. The internal hardware from the 5c is from 2012/2013 and it's too old to be upgraded further. It's CPU doesn't support 64-bit which would be a requirement for iOS 11. Sorry, there is nothing you can do, except maybe to contact your carrier.

Or to buy a newer iPhone generation.