One of my toilets is not flushing properly. The water is clear but very low in the bowl. It acts if it is clogged but using a plunger does nothing. It has been below zero for several days & the toilet is on an outside wall. My other toilet (which is on an inside wall) is fine.

Be surprised to see a frozen drain line since the decomposition gasses keep well above zero. A low bowl is typical of a clogged drain.