I cannot get the Xbee module to produce a signal for the RSSI and could not find anything mentioning that this should not work. Module is updated to latest firmware, configured to default settings except the two noted below.
XB3M1 - Digi XBee3 Cellular LTE-M/NB-IoT Global
Firmware version 11410 (Newest)
AP - Bypass Mode 
P0 - RSSI PWM0 Output 
No signal is produced on DIO10 ( Pin 6 ). Tested in XCTU, Fluke DMM, Digi breakout, AT+COPS=? network scan as well as successful Linux PPP connection.
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.