After replacing the defrost timer, I still have the same problem a week later (freezer not defrosting/fridge warm) so now I want to check continuity of the defrost thermostat. I am new with a multimeter so my first question: Can I test the thermostat without removing it? Put the 2 multimeter leads onto the 2 bare wire leads of the thermostat (unplugged of course)? When you say "open circuit" does that mean a zero ohm reading?
And question 2: I see the thermostat on the parts diagram on this site going into the evaporator. Is this located in the freezer behind the back panel? I can't tell from the diagram. Thank you.

Here are your parts
Replacement parts for FRIGIDAIRE FRT22TSEB2 Frg(v1) | AppliancePartsPros.com

You may want to start from scratch, just in case you received a defective timer.

First remove the evaporator cover in the freezer so you can see the coils.
Do not let them de-ice.
If they are heavily iced/frosted over you have a defrost problem.

If yes.
Manually force a defrost cycle by turning the defrost timer cam (Item 23 in Section 5) till the fans and compressor turn off.
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the timer needs replacing, re: it is probably stalling during it's rotation so is never getting into a defrost cycle.

If not on.
Unplug the unit.
Remove the wire for one side of the heating element (Item 4 in Section 6) from the wiring and measure it for continuity, usually around 20 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat (Item 21 in Section 6) and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.

If both the above are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.
[COLOR="DarkRed"]
Can I test the thermostat without removing it? Put the 2 multimeter leads onto the 2 bare wire leads of the thermostat (unplugged of course)?[/COLOR]
You could but it is always best to disconnect at least one side of any device you are measuring for resistance. This prevents toy fram reading an alternate/parallel circuit path.

[COLOR="DarkRed"]When you say "open circuit" does that mean a zero ohm reading?[/COLOR]
No. Zero ohms is closed. Infinite ohms is open.
With meter on leads not touching what shows up on your meter is infinite ohms.
Short the leads together, what shows on your meter is 0 ohms (closed).
When checking for a closed contacts/thermostat use the most sensitive meter scale.

Thanks Denman. You mentioned a live test of the timer. I assume this is done when plugged in. Does the fridge need to be in defrost mode (turn timer to click) during the test? Where do I put the multimeter leads when doing this test as all the connections are in rubber boots? Thank you.

After problem returning with a new defrost timer, I did what you recommended and bypassed the defrost thermostat. I didn't even need to cut the wires as the male/female boot connectors were the same on the heater and the thermostat. The heating element came on & started to do its job. Faulty thermostat, even though I got good readings on it (0 ohms when frozen). Thanks for your help, part is on its way.