Section 3 of the 20th Amendment specifies that( for whatever reason)neither the President or the Vice-President is elected in time(January 20th), then the Speaker of the House becomes acting President until the House selects a President from the top three vote winners, OR the Senate selects a Vice-President who will then replace the Speaker as acting President until the House selects a President.
Delay posed by Mail-in voting presents a Constitutional Crisis which the 20th Amendment cannot remedy.
You are not familiar with Trump's current powers under Executive Directive 51 apparently. Look it up, and get back to us.
FReegards!