Hi @Saylent_N_A, welcome to the forums.
Server logging should also show any failure reasons, search/filter on "c":"ACCESS".
A user can be created to include extra authentication restrictions on the client ip address(es) and server ip address(es).
Use the db.getUser('username', {showAuthenticationRestrictions:true}).authenticationRestrictions command to list any.
// example
db.getUser('foo',{showAuthenticationRestrictions:1}).authenticationRestrictions
[ { clientSource: [ '127.0.0.1/8', '192.168.1.1/24' ] } ]
Another possibility us that the port is being forwarded to another mongodb instance?