[OnePlus 3 & 3T Cross Device Development] AlkaliV2:
I'd say 'no' again and offer a large quantity of evidence. Hopefully you'll actually read it since I put a lot of time into finding it all. But if not, other people might get bored and learn about signal strength:

Understanding the (R)adio (I)nterface (L)ayer Used in AOSP

https://www.slideshare.net/ssusere3af56/android-radio-layer-interface

OxygenOS Official Github android_hardware_ril

https://github.com/OnePlusOSS/android_hardware_ril/tree/oneplus/QC8996_N_7.1/include/telephony

LineageOS Github android_hardware_ril

https://github.com/LineageOS/android_hardware_ril/tree/cm-14.1/include/telephony

AOSPA android_hardware_ril

https://github.com/AOSPA/android_hardware_ril

The stuff above looks similar because it is. OOS provided the needed vendor files and custom ROMs use those files to get their ROMs working on the network. The same way they took the dash charge files and added them to implement that feature.

ROMs can do nothing to physically improve your existing connection; that all happens when the hardware and the baseband handshakes with your tower. Your cell signal is your cell signal--no amount of bars, real or fake, will change that. The numbers you see in the settings menu are the mathematical calculations handled by Android of estimated signal strength provided every few seconds converted into signal bars for you see in the upper right corner. This means that when your baseband implementation decides to leave a tower no ROM can prevent that or change how it interfaces with the tower to improve connection. Vendor device driver updates (Qualcomm/Oneplus) for the modem and baseband are the only things that can improve your signal on a device which is why new modems updates come rolled into firmware packages for the Oneplus 3/3T. And before you go into, "Global, GSM, CDMA" selection, those are specified selection settings that instead of polling the baseband for all possible signals it narrows the field to just the ones you want. So in the U.S. I only want to use CDMA/LTE while overseas they want to use GSM/LTE. Those settings do not change how much signal you have, only what signal is searched for at a ROM level. Don't you think if a ROM was able to provide significantly better signal they'd list it in the features section or in the title of the thread? I know I would.

More Links Explaining Your Modem and Baseband Determine Signal Strength and Tower Switching

This one talks about custom ROMs - https://ohthehugemanatee.org/blog/2015/04/04/how-and-why-to-update-the-modem-slash-baseband-firmware-on-a-samsung-phone/

This defines baseband - https://www.quora.com/What-do-baseband-version-and-kernel-version-mean-in-Android

A simple explanation - https://android.stackexchange.com/questions/70/what-is-radio-firmware?noredirect=1&lq=1

Side note: You also misunderstood my battery statement. You battery voltage (and current) is what allows your phone to stay powered on. Android constantly monitors that voltage and when it drops below a certain threshold it shuts the phone down. A lot of people experience an issue in Android where their battery will tell them they have 5-15% battery remaining but the device shuts down anyway. This is because Android can't always calculate the correct percentage in phones with older batteries that no longer have the same overall capacity. When the battery voltage is too low to sustain operation Android shuts down no matter what percentage your battery reads.