Express router GET endpoint gives Not Found error

Question

This is my app.js

app.use('/v1/abc/', router);

Then secondary level

router.use('/:primary_id/def',defServices);

Final service level

     class defServices {
      constructor() {
        const router = new express.Router({ mergeParams: true });
        router.get('/', this.func1);
        router.get('/:pid',this.func2);
        router.put('/', this.func3);
  
        return router;
      }
    
      async func1(req, res) {
     //code goes here 
     }
  }

export default new defServices();

My request

GET  http://localhost:3000/dev/v1/abc/48/def

I'm having an issue with the first GET endpoint not reaching and getting 404 responses, but the second, GET endpoint and third PUT endpoint works fine. Furthermore, this implemented in serverless-offline