Feature phone with 3G modem capability used to work under 1.18 but no longer works under 1.20

[Alex Villacís Lasso]

I have this old phone, a Samsung E3210B pre-Android feature phone that I 
wish to use as a fallback option when (more frequently than I would 
like) my broadband internet service goes down. When using ModemManager 
1.18.x, I could plug in this phone, and it would expose a single 
/dev/ttyACM0 port for all interactions including AT commands and PPP. 
Then ModemManager could connect to the internet using the port as 
normal, using the "generic" plugin. Now, using ModemManager 1.20.6 under 
Fedora 39, the same phone fails (with a timeout) to establish a 
connection using ModemManager. However, it does setup a proper 
connection using wvdial from the command line, even under Fedora 39.

After a while, I managed to perform a git bisect on the code, and 
pinpointed the first commit that breaks my phone, to commit 
213cd81b3ade35024e2d702e2726273f00344185 "iface-modem-simple: wait for 
packet service 'attach' state in ConnectionStep". However, I am now at a 
bit of a loss at understanding the supposed purpose of this commit. Full 
details are at this bug report:

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/743

The patch no longer cleanly reverts under 1.20.x, so I cannot just rip 
it off...