No incoming layer 3 packets with Huawei ME909u-521 on LTE

I'm not sure it's specificaly a ModemManager issue, but using a Huawei ME909u-521 (12d1:1573) on a PC Engines apu6 works great when using a 3G protocol, but fails with a 4G protocol (LTE): then, no layer 3 packets are apparently sent back to the cdc_ether interface (though the RX count given by modem-manager-gui does increase, eg. with a ping -i 0.1). In other words, outgoing packets are actually sent by the host and received by targets over the Internet (I checked ICMP and UDP), but their answers apparently never reach back the host (except for ARP packets from the gateway).

Here's the log when using mmcli --modem 0 --set-allowed-modes=3G: 3G.log

and here's when using mmcli --modem 0 --set-allowed-modes=4G: 4G.log

Please, do not hesitate to ask me for more debug logs / command outputs if that may help you figure out what's happening. This simple change from 3G to 4G triggers the problem, and coming back to 3G solves it. Hence I think it rules out any firewall or sysctl misconfiguration on my side.

Figuring out that the 3G/4G switch was solving/causing this problem, was not easy, thanks to the bad weather which made only 4G out of reach for a brief time :) but I'm now out of clue on how to debug this further. (except maybe asking my ISP, but they proved themselves unsupportive by the past).

Thanks in advance for any help.

$ uname -a
Linux aubergine 5.15.61 #1-NixOS SMP Wed Aug 17 12:24:32 UTC 2022 x86_64 GNU/Linux