Is it a sin to use the company's internet at home for personal purposes after the monthly subscription has expired?

If using the password of the work device on your personal device does not affect the work, then there is no harm, but this should be with the permission of the official at the company, because it is considered benefiting from its assets and may affect the line speed. It is more prudent to refrain from doing so, but if it is done sometimes out of need, it is not forbidden.