Fallback on Android only works one way.

These options allow the technologies. 2G is just 2G and nothing else. With 3G it’s 2G/3G with 2G as fallback option. Like that you have 2G/3G/4G/5G and disabling 2G switches to 3G/4G/5G.

In this case it just stays on 2G because there’s no way to subtract it into nothing. Although perhaps I just influenced it by observing it ¯⁠\⁠_⁠༼⁠ ⁠•́⁠ ͜⁠ʖ⁠ ⁠•̀⁠ ⁠༽⁠_⁠/⁠¯

Edit: Falls back to the last valid option in the background. If 2G only was first, it ends up using that, if this was toggled first, it returns from previous selected network type.

That’s how I got that, xD. Android doesn’t otherwise list 2G as an option anymore, though it can still display it.

I wish I could deselect 3G however, since that isn’t a thing in my country anymore. (While keeping 2G)

Even more advanced (on MediaTek) stuff is in *#*#3646633#*#*. It’s called engineer mode, and typically gets removed in OS shipped to customers, but some manufacturers just don’t do that (smaller Chinese). There I can even select specific network bands, or even specific cell tower by EARFCN and Cell ID, as well as tinker with settings I rather won’t touch because I don’t understand them. And for example, manual MTU setting can persist a factory reset, so not everything may be resettable.

Even funnier, and probably part of the reason for removal, there’s Tx tests. For WiFi and cellular. Unfortunately, it only works for 4G on my current phone, and it blasts full power no matter the setting. But on my other MTK phone I can select 2G, 3G or 4G, frequency to transmit on, bandwidth, modulation, power level, type of data to transmit and some other settings.
Welp, that one sounds illegal.
I checked it with my SDR, it works. (The lowest power only seems to reach a few cm).
Lastly, based on screenshot found on XDA, some variants allow easily changing IMEI via GUI, or at least using AT commands. Also likely illegal.

I find it very useful for the band selection.