HP lt4120 failing to make single-stack connections on LTE using MM

[Tore Anderson]

* matthew stanger

>     it only seems to happen with the single-stack IP types (both IPv4- and
>     IPv6-only), never with dual-stack.  
> 
> From what I've seen many carriers (and subsets of towers) don't support every IP type, especially IPv6.

In my case, that's not the issue. I'm using SIM cards from carriers that
do support IPv6/IPv4v6 in (v)PLMNs that do support it. As I mentioned, it
does work fine when using «qmicli --wds-start-network» and or if I try
using other UEs (phones).

It's only when using ModemManager (and, by extension, NetworkManager) that
it gets stuck sometimes. 

* Aleksander Morgado

>     $ sudo mmcli -m 0 --reset

Thanks - I had overlooked that one, but it doesn't quite work:

error: couldn't reset the modem: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Unsupported: Cannot reset the modem: operation not supported'

(Similar result with --factory-reset=123456.)

That said, now that I thought to search for «factory» I realised there's
«qmicli --dms-restore-factory-defaults». That's probably even better than
just a powercycle, considering my goal is to have a way to bring it back
to a clean slate. Luckily the required code was easy to guess: 000000.

However it says «Device needs to get power-cycled for reset to take effect».

«qmicli --dms-set-operating-mode=low-power» followed by
«qmicli --dms-set-operating-mode=online» does not do the trick, and if I
use «offline» instead of «low-power» I can't bring it back to «online»
again as it fails with «QMI protocol error (60): 'InvalidTransition'».

Are there any other ways to powercycle a modem?

Tore