The country where you work is your reference country in terms of social security because contributions are usually (but not always!) deducted there directly from your salary.
You therefore have to register with a Belgian mutual insurance company.
In Belgium, the mutual insurance company is the reference organisation for all the healthcare aspects of social security (reimbursement of healthcare, payment of benefits in the event of incapacity for work, etc.). It is up to you to follow the procedures to register and not up to your employer, as is sometimes the case in certain countries.
Are you starting work in Belgium? Here arecross-border-worker-abroad the procedures you need to follow to receive Belgian social security protection.
If you live in Luxembourg, document S1 is replaced by document BL1.
Are you in the opposite situation? Find out what procedures you need to follow if you live in Belgium and work abroad.
What procedures to follow if you come to work in Belgium but live abroad.