Neither as an employee nor as a self-employed person. There are no exceptions. The only way to obtain residence is to follow the correct procedure before you arrive in Belgium. If necessary, you can return to your country of origin or a country where you have legal residence and start the procedure from there. You’ll find out how the procedure works here.
There are limited alternatives in Belgium for legalising your residence. The best thing to do is get in touch with a specialised service and explain your situation. They will correctly inform you correctly whether there are options for your case. You can find specialised services here.