Hi,
I use a Raspberry Pi 3B+ with the latest rasbian and a Huawei E3372h 4G LTE Sitck in cdc_ether mode.

The stick presents itself as ethernet interface, has IP 192.168.8.1 and gives me the IP 192.168.8.100 via DHCP.

I can send packets of any size but only receive packets that are 1472 Bytes or smaller.
As soon as I receive a packet bigger than 1472 it is actually 0 bytes long according to tcpdump.

I have uploaded a pcap file of TCP traffic while running "sudo apt update" which is hanging on "Waiting for headers" here: https://filebin.net/32ipqghvpm7ejtv2

I also have tried diabling gso and gro for the E3372 interface, but it does not make a difference.

Can anyone help me finding the issue?

I have set the TCP MSS to a fixed value via iptables and now I can get TCP connections:
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1400

So it seems to be incoming TCP fragmentation is not working.

How can I debug where the problem begins so that i can report a bug to the affected component?

Edit:
This seems to work for ICMP, and a few HTTP servers, but not for every TCP connection.
Why does this even affect ICMP messages?

You can set smaller interface MTU. What is your interface MTU?

1 members found this post helpful.

I think MTU also. There is a way to test the limit using ping if you wish. Been a while since I've seen that but modems were like that and I assume this is connecting like a modem.