MONGO_FDW on Postgres user privileges

Setting up MONGO_FDW in Postgres for the first time.

First step is to add the extension after connecting as the default postgres user:

CREATE EXTENSION mongo_fdw;

The application user is app_user, so I grant this:

GRANT CREATE SERVER TO app_user;

I then connect as app_user and issue:

CREATE  SERVER "app_rem_db"
    FOREIGN DATA WRAPPER mongo_fdw
    OPTIONS (
        address 'localhost',
        port '27017'
    );

But I get the error:

ERROR: foreign-data wrapper "mongo_fdw" does not exist

If however I run the CREATE SERVER as postgres it works.

How do I allow other users to create the server?

Continuing with the postgres user though, I next create the USER MAPPING:

CREATE USER MAPPING FOR app_user
    SERVER "app_rem_db"
    OPTIONS (username 'app_user_pg', password 'hello123');

If I then again connect as app_user and try to create a foreign table I get (The mongoDB has a database mongo_test and a collection testcol therein):

CREATE FOREIGN TABLE mongo_testtab
(
    data JSON
)
SERVER "app_rem_db"
OPTIONS (database 'mongo_test', collection 'testcol');

I again get an error:

ERROR: server "app_rem_db" does not exist

If I switch back to postgres user I can create the foreign table.

But then when I try to query the table I get:

select * from mongo_testtab;
ERROR:  relation "mongo_testtab" does not exist
LINE 1: select * from mongo_testtab;