Registration if I work in Belgium through a foreign company under 180 day rule (LIMOSA)

do you know this website already?https://www.international.socialsecurity.be/working_in_belgium/en/limosa...
it's explains clearly
a) you are seconded from a (registered, your taxes & social security paying in that country) company
b) for registered self-employed ..thus again, paying taxes & social security in your registered country

...and you don't register here as you have in both cases 'a home base'.
LIMOSA has nothing to do with the 180 days rule in a first view but a lot with the confirmation to the BE authorities that somebody works under social security laws / minimum wages etc. of a registered home base country.

Thus means also (IMHO not professional) you will NOT get INTO the BE system via Limosa.

Ask your employer - they handle the paperwork.