Here’s what I’d do.
1 Make sure the inner cable is long enough and not broken. When it’s seated in the driven gear it should stick about a quarter of an inch out of the speedo end. You can also pull the inner cable out of the housing to ensure it’s all in one piece.
2 Make sure there’s no kinks in the cable run or spots where it might get pinched. That can cause erratic operation.
3 Make sure the cable is operating. With the rear wheels off the ground and the fronts chocked unhook the cable from the speedo and see if it turns when the car is running and in gear. A piece of cardboard or heavy paper pushed onto the end of the inner cable will make it more apparent.
4 If it isn’t turning make sure that the cable is fully seated in the transmission. If it is, remove it and check that the cable end is a snug fit in the gear on the transmission end of the cable.
5 If it is turning check that the cable fits the square drive hole in the back of the speedo and can’t rotate in it. If it can either the speedo hole is worn out or the cable is the wrong size. Betting money would say that the cable is wrong.
6 If all of this looks ok then it may be the speedo has failed. It’s a magnetic drive for the pointer, the odometer and trip meter are geared. If they are working while the pointer doesn’t, it’s the gauge.
Good luck.