I think you should check your manual, that switch labeled 'Tank Heater' might be for heating your water and waste tanks in cold weather. Often the water heater will have a 120 v switch that is lighted when on to turn on the element in the water heater IF
there's water in the tank AND
the 'hidden' switch to turn on the 120 v element in the water heater compartment is switched on. Remove the WH cover outside and see if there's a switch there for the 120 v element.
The gas part of the water heater is often turned on by a 12 v switch that appears to be on the lower left of your levels panel. It also usually has an LED to show when it's on. When turned on, the blower fan should come on immediately and then the ignitor and gas should come on.
Look at your water heater manual for more instructions. If you don't have one, look on the heater (behind cover panel) and look for brand and model number. Most manuals can be found by a Google search and going to manufacturer's website. It also probably has a troubleshooting guide and parts list included.