How to give readonly permission to postgres database role?

Hi I have postgres database and I have created user like below

CREATE USER username WITH PASSWORD 'your_password';
GRANT CONNECT ON DATABASE database_name TO username;
GRANT USAGE ON SCHEMA schema_name TO username;
GRANT SELECT ON ALL TABLES IN SCHEMA schema_name TO username;

Now I am using this user to execute Stored procedures. I have executed below statement

GRANT EXECUTE ON ALL PROCEDURES IN SCHEMA myschemaname TO myuser;

In my stored procedures I have select, insert, update and delete statements.

whenever I execute Stored procedures and if stored procedure contains any insert,delete update statements its throwing error

42501: permission denied for table tablename

If i have only select inside the SP then no issues only problem with insert,update and delete. So my question is I have created user for the purpose of executing stored procedure only and i do not want to give update,delete and insert access to any table. So what should I do to execute inset,delete inside SP? Can someone help

Create the procedures with SECURITY DEFINER which specifies that the function is to be executed with the privileges of the user that owns it.
Check here.
Make sure that the owner of the procedures have required privileges of all operations on resources within the proc body.

CREATE OR REPLACE PROCEDURE  procedure_name()
AS
$$
BEGIN
  -- UPDATE;
  -- DELETE;
  -- INSERT 
END;
$$ LANGUAGE PLPGSQL SECURITY DEFINER;