Since Mr. Trump has sworn to uphold COTUS, he needs to declare this election null and void, and order a new one.
If States insist on mail-in ballots and the same problems persist, Mr. Trump can remain President until December 14th and the Amendment 12 solution kicks in:
But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice. (Amendment 12)
Trump is President until Jan 20th regardless of the outcome of this election. Dec 14 is just when the EC meets in each state.
If the election disputes have not been resolved when Congress counts the votes, then it will go to the House.