A work permit, include possible CUSMA/NAFTA facilitation, is usually only possible if you have a Canadian employer or you are self-employed in Canada.
If your principal purpose is to temporarily visit Canada and your activity does not qualify as working in Canada, you may carry out such activity without a work permit.
What kind of activities are not considered to be “work”?
An activity which does not really 'take away' from opportunities for Canadians or permanent residents to gain employment or experience in the workplace is not “work” for the purposes of the definition.
Examples of activities for which a person would not normally be remunerated or which would not compete directly with Canadian citizens or permanent residents in the Canadian labour market and which would normally be part-time or incidental to the reason that the person is in Canada include, but are not limited to:
[...] long distance (by telephone or Internet) work done by a temporary resident whose employer is outside Canada and who is remunerated from outside Canada; [...]
U.S. citizens do not require prior permission (visa or eTA) before seeking admission into Canada as a temporary resident. Your application for admission will be considered at the border.
Temporary residents without work or study permit (i.e. visitors) are usually admitted for a six-month period. You may extend your visitor status without strict limitation, however you are required to convince an agent that your purpose in Canada is temporary and you will leave Canada by the end of your authorization. For example, if you do not have a home to return to, this may count against you. If you tell the officer that you intend to move to (instead of visit) Canada, it may also negatively impact your application.
Other factors other than immigration laws
Note that you can still be liable to taxation in Canada if you become a tax resident of Canada (which is independent of your immigration status), which depends on the situation, may also create additional responsibilities for your employer. Taxation obligations may already exist for your employer if you carry non-incidental employment duties in Canada.
Even if for the purposes of the federal immigration law your activity may not be "working", provincial employment standards (minimum wage, break time, workers' compensation etc.) may still apply based on physical location of the employee. That is also partly why many businesses would rather not employ you as a remote employee (instead of as a self-employed contractor or an employee of a local agency) in another country.
You also would not qualify as a visitor for provincial healthcare coverage. Your U.S. or other insurance may have exclusions if you spend too much time in Canada.