On my gmail account I have the settings for "send mail as" then I clicked on "always reply from my default address (currently
xxxxx@charter.net
)".
This is the setting for the account when sending messages via webmail using a browser only which has no effect when sending messages with the account from the iPhone's email client. Sending messages with an account via webmail does not use an SMTP server.
The SMTP server used with an email client to send a message with the account does not affect what appears as the sending email address for the account. This is determined by what is entered as the email address for the account which is separate from the SMTP server.
On your iPhone, go to Settings > Mail. Select your Gmail account.
Under Account Information for the account is Name, Address and the account's Description which is the name for the account.
What you enter in the Name field and Email Address or Address field is what appears as the sending name and email address for an account.
Enter your Charter email address in the Address field for your Gmail account preferences and then try sending a message.
Gmail's authenticated SMTP server may prevent this and if so, you can set up your Charter account with the iPhone and if you are able to receive messages with the account, enter Gmail's authenticated SMTP server settings as the SMTP server used to send messages with your Charter account which Gmail doesn't prevent.