Suburban NT-20seq furnace will not light at all.

how old is the furnace?   I had a similar problem with mine when I first got it and it turned out to be bugs in the combustion chamber and the exhaust. not to say it is your issue but its worth a check.  I pulled the igniter and burner, used a tiny vacuum attachment for computers to clean that side, and I blew compressed air from the outside through the setup, and did this several times until no more evidence of bugs showed up.