When configured for bypass mode, complete AT command control of the underlying cellular component (u-blox SARA-R410M in this case) is handed over to the user. This means that the XBee firmware is incapable of communicating with the cellular component itself for any purposes (reading signal strength, changing APN, doing IP or SMS activity... anything).
This is at least hinted at here: https://www.digi.com/resources/documentation/digidocs/90001525/concepts/c_mode_bypass.htm
"When Bypass mode is active, most of the XBee Cellular Modem's AT commands do not work. For example, IM (IMEI) may never return a value, and DB does not update."
When we say IM "may" never return a value, we actually mean it "will" never return a value.
If you need RSSI PWM functionality, you will need to use the regular XBee firmware functions like transparent/API mode and/or MicroPython programmability.