Re: "why can i text everyone on my phone but one person?"
Maybe that one person uses an Android phone which requires the use of cellular data for texts etc.?
(Apple to Apple devices just use wifi for iMessages.)
Charges may or may not apply, depending on your provider, when you turn on and use cellular data for Messages:
See how to: View or change cellular data settings on iPhone - Apple Support
Eg: says:
"Set cellular data usage for apps and services
Go to Settings > Cellular, then turn Cellular Data on or off for any app (such as Maps) or service (such as Wi-Fi Assist) that can use cellular data.
If a setting is off, iPhone uses only Wi-Fi for that service."
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Re: i’ve tried ... ... turning off my wifi but nothing works :
sometimes it’ll work periodically but then stop working again"
If wi-fi assist is turned on, messages may go via cellular if there is no wi-fi signal available.