Strangely low uplink speed only when the modem is connected to an SFP+ module

[Chunchunmaru_]

First of all let me explain my home network setup.

 

I have this 5G Modem that connects to the internet on a remote place, a ZTE MC801a with internet speeds up to 600Mbps of Download and 60Mbps of upload, with 2x 1 Gbps RJ45 ports
On this modem 10m further there connected this "chinese" unmanaged switch, with 4x 2.5 Gbps ports + 2x SFP+ 10Gbps Modules.
The switch has 3 ports used and is the main access to the entire network.
The ports used are three, two of the 4x 2.5Gbps and one of the 2x SFP+ 10Gbit ports:

- 2.5 Gbps RJ45: My 2.5Gbps PC network card

- 2.5 Gbps RJ45: My 2.5Gbps access point

- 10 Gbps SFP+ RJ45: My 1Gbps negotiated 5G modem with 10m of cat5e cable

Long story short, ONLY with my desktop PC, and ONLY when the modem is connected to one of the two 10Gbps SFP+ ports, the upload speed tests are somehow capped at 20Mbps rather than 60Mbps only from that desktop. Download looks normal.

The funny thing? Negotiating manually the speed of the PC from 2.5Gbps to 1Gbps on my desktop, the upload speed goes back to normal.

I have tried the following:
- Connecting the modem to the RJ45 2.5Gbps port, the issue is fixed

- Trying different OS, issue present on both Linux and Windows

- Trying a speedtest from the 2.5Gbps AP, issue not present here.

- Resetting all devices

- Trying to change ethernet related power saving flags

Since those SFP+ modules are expensive, I'd like to know what the hell is going on... I can't really even remotely figure out the issue here.

It could be a compatibility problem with the PC network card and the router, but only when the switch is connected with the SFP+ 10Gbps port to the modem... Other devices do not have the same issue. Still, doesn't make any sense. I also tried shorter cables, the ethernet cables going to both PC and modem are 10m long each other, and the upload speed to other devices in the network are just fine.

I tried putting a laptop which has a 1gbps nic like the modem, with iperf on the SFP+ 10Gbps port and the results look normal.

Edited December 16, 2023 by Chunchunmaru_
Added image