If you are selecting 'SSO2' as SSO mechanism (you may remove Technical User one), then your odata producer should return MYSAPSSO2 token. For that you have to update HTTP/HTTPS authentication provider under SAP_SSO2 security profile with below parameter
SSO Cookie name: MYSAPSSO2
For initial on-boarding you have to pass BACKEND credentials in authorization header.
I would like to understand how exactly you have developed UI5 app using SMP3 as you mentioned above
Regards,
JK