sideways cut on a router table

Not quite sure what you mean, but if you're going to do the cut on the table, you will need to support the 2x's overhanging end. One way to do that is with a work support stand that you adjust to the same height as the table top. The Home Depot has this stand at $25. HDX 43 in. Steel Roller Stand with Edge Guide-AC43 - The Home Depot

You'd need to set up a fence so you can keep the end of the 2x the same distance so the cut will be 90 to the board.

Personally, I'd rather do that freehand with a straight edge clamped to the board as a guide, and a couple of 2x on each side of the cut to keep the router stable. Leave a little space between the pieces, a little wider than the bit's diameter. Use a 2x for the straight edge, and clamp all three pieces down to either a work table or a piece of ply on saw horses. You'll still need the work support stand if the long end of the 2x is very long.

I have marked the height on the sliding support stand for the height for the table saw and the sliding miter, plus marked the floor for stand location for each tool. Makes setup easier. This particular stand also folds flat and tucks out of the way nicely.