I'll confirm this works very well for 3G (600-900MHz) although I cut the hot off the plug out of paranoia when using one this way. Some of the 4&5G bands are up above 3GHz where it won't work so well.
Since your WiFi signal, however is almost exactly the same and there are many apps that will show and record signal in dB (as described in TFA). You could measure while the door open/closed if you're really careful not to turn it on. The lithium battery fire could respond very badly to water!
Depending on your region, LTE could be using much higher frequencies. Microwave ovens use a fixed frequency and the cage is designed for that. Considering this I would say it's still an effective test.
Hint: If you're ever doing radio work in the field and need an instant Faraday cage a microwave oven is a good candidate. Turn the power off first to reduce the risk of accidentally irradiating your device, but leave it plugged in to get an earth connection to the shield. (This is how I know an LTE connection can be maintained in a microwave oven.)