Connecting to a new RDS instance from an EC2 instance
Check my server logs in MySQL Workbrench or Command-line Could not acquire management access for administration. 


Right, you can't do that with workbench and RDS because that requires shell access, which RDS does not allow.  The logs are accessible through the console and API.


  Remove rdsadmin or Revert the asingeded roles and privileges 


Right, you can't do that, because rdsadmin is the account that the RDS infrastructure uses to manage your instance.  You don't control it.


  Grant DBA/MaintenenaceAdmin/ProcessAdmin or Replication Admin to WPuser Error changing account WPuser@%: Access denied for user 'WPuser'@'%' (using password: YES)


The problem here is threefold.  

The "roles" in workbench were a ridiculous idea on the part of Oracle because they do not correspond to real structures on MySQL... They're just fancy packaging for permission presets and you'd be better served to ignore that functionality and understand how the permissions actually work.  

The DBA role doesn't work on RDS because that includes the SUPER privilege, which RDS does not provide, so that one wouldn't work anyway, even if you hadn't made the final error:

You appear to be trying to give "WPUser" permissions... while you're logged in as "WPUser."  You can't grant permissions to yourself.  You also can't grant permissions you don't hold.  If WPUser is the master user for your RDS instance, it already has all the permissions it can have... though you shouldn't use your master user for your application's access to the database.


  I added the Elastic/Private and even Public IP address for the EC2 instance to the VPC security group


That isn't likely the problem, here.  If the security group isn't right, you'll fail to connect, after a lengthy timeout, and the error code will not be "access denied." 

Also potentially incorrect, depending on what you are using as 'ENDPOINT':

GRANT ... ON WIKIdb.* to 'WIKIuser'@'ENDPOINT';


If 'ENDPOINT' is the RDS endpoint, this is incorrect.  You can never connect to RDS from the RDS endpoint, or even from 'localhost'.

The final argument there needs to be the IP address of the server where the wiki code is running, or a wildcard for the netblock, like '172.31.%'.