TN does not apply to work in Canada. You would need the TN only when working in US. If you and your employer are CHOOSING to have you work remotely in Canada, then you should not be on US payroll, but on either a Cdn payroll set up by the employer or an agency, or be self-employed contractor.
BUt if you are getting a TN, to work physically in US, then you are indeed allowed to cross the border. You would of course be subject to quarantine rules on both sides if the border, including regular covid testing nd isolation when returning to Canada. So it may only be practical to choose to be on one side of the border and stay there.