Hi Leko_R,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with changing your default mail account on your iPhone. We're happy to help.
If your iCloud mail is enabled you should be able to view the option to select from Default Accounts, as shown here: Using your @icloud.com email address. We recommend navigating to Settings > [your name] > iCloud and check that Mail is turned on.
If turned on and the issue remains, ensure you can successfully send a message from your iCloud account. The steps in the section "If you need help sending or receiving mail on your iPhone, iPad, or iPod touch" from the article here can assist: If iCloud Mail isn't working
Should the email send as expected, test the method in the section "Send an email from a different account" from the guide here as well: Address and send email in Mail on iPhone
Additionally, you can test a normal restart of your iPhone: Restart your iPhone
Cheers!