I've used to look for smaller Chinese brands that don't bother removing factory test tools, but after discovering baked-in malware, I don't really feel comfortable with that.

Examples

This is an example from MediaTek Engineer Mode:

And here's an example of selection made with mmcli --set-current-bands (may work on Linux phone OSs like PostmarketOS):

(PostmarketOS may use ModemManager: https://wiki.postmarketos.org/wiki/Modem)

Use cases

Particularly the 800MHz LTE band (B20 and eutran-20 above) tends to be overloaded, and also typically has less allocated RF bandwidth.

Automatic selection may or may not work as desired. For example, I find 800MHz to be favored indoors in my location.
In worst-case scenario, I can use this to jump from B20 to B7, the latter of which does carrier aggregation (20+20MHz) with the carrier (provider) I use, boosting me from 15Mbps to 130Mbps.
This will cost some extra battery use, but meh.

Another is with carriers that have agreement for network sharing with another carrier as to provide coverage extension.
For example, when I used Swan Mobile ("4ka") in Slovakia, the carrier had own base stations in B3 (and thus highly preferred) and coverage extension from Orange on B1 and B8. Since this was no longer implemented as roaming, this was the only method of manual control.

And they're shitty enough to have FUP on something you typically can't control.

Existing solution

There's an app called Network Signal Guru: https://play.google.com/store/apps/details?id=com.qtrun.QuickTest

But I don't know how trustworthy it is to give root access to.