Activate oplog on a standalone instance and to dump

I want to turn on oplog in a standalone instance in order use mongodump --oplog on it however I fail to achieve that.

I follow 2 ways without success:

After trying, I have this configuration:

rs0:PRIMARY> rs.conf()
{
	"_id" : "rs0",
	"version" : 1,
	"protocolVersion" : NumberLong(1),
	"members" : [
		{
			"_id" : 0,
			"host" : "smartshape.io.test:27017",
			"arbiterOnly" : false,
			"buildIndexes" : true,
			"hidden" : false,
			"priority" : 1,
			"tags" : {
				
			},
			"slaveDelay" : NumberLong(0),
			"votes" : 1
		}
	],
	"settings" : {
		"chainingAllowed" : true,
		"heartbeatIntervalMillis" : 2000,
		"heartbeatTimeoutSecs" : 10,
		"electionTimeoutMillis" : 10000,
		"getLastErrorModes" : {
			
		},
		"getLastErrorDefaults" : {
			"w" : 1,
			"wtimeout" : 0
		},
		"replicaSetId" : ObjectId("628b94350b78e951119f508c")
	}
}
rs0:PRIMARY> rs.status()
{
	"set" : "rs0",
	"date" : ISODate("2022-05-23T14:29:01.664Z"),
	"myState" : 1,
	"term" : NumberLong(1),
	"heartbeatIntervalMillis" : NumberLong(2000),
	"members" : [
		{
			"_id" : 0,
			"name" : "smartshape.io.test:27017",
			"health" : 1,
			"state" : 1,
			"stateStr" : "PRIMARY",
			"uptime" : 1541,
			"optime" : {
				"ts" : Timestamp(1653314744, 2),
				"t" : NumberLong(1)
			},
			"optimeDate" : ISODate("2022-05-23T14:05:44Z"),
			"electionTime" : Timestamp(1653314614, 1),
			"electionDate" : ISODate("2022-05-23T14:03:34Z"),
			"configVersion" : 1,
			"self" : true
		}
	],
	"ok" : 1
}

Context:

root@smartshape:~# mongod --version
db version v3.2.22
git version: 105acca0d443f9a47c1a5bd608fd7133840a58dd
OpenSSL version: OpenSSL 1.0.2n  7 Dec 2017
allocator: tcmalloc
modules: none
build environment:
    distmod: ubuntu1404
    distarch: x86_64
    target_arch: x86_64

Not working means what error are you getting?
mongodump with --oplog works only for full DB
Please show the error logs from your mongodump

My bad, I forgot the result.

Here is the result after activating the oplog and update/insert some document in MongoDB collections, oplog are always empty:

root@smartshape:~# mongodump -v  --oplog --out ./backup
2022-05-23T09:25:47.804+0000	getting most recent oplog timestamp
...
2022-05-23T09:25:47.828+0000	writing captured oplog to 
2022-05-23T09:25:47.828+0000		dumped 0 oplog entries

How big is your db?
How long it takes to dump?
Any activity going while backup runs?
Do some activity and see