Gigabit Ethernet on ZimaCube

I have ZimaCube (not PRO) which has two Ethernet ports at the back. AFAIK these are 2.5 Gbps ports, but should be backward compatible with 1 Gbps Ethernet, right?

Now, when I connect Ethernet cable to my docking station, I’m getting 1Gbps link (checked dmesg) what’s fine - Ethernet works. The problem starts if I put my 1Gbps switch in between. I connect switch to the docking station, there’s link; I connect my old NAS built on Gigabyte ITX motherboard, it works.

When I connect ZimaCube to the switch there’s no link. The switch flashes orange LED once and connection is not establish. What’s pretty bad as I need both NASes connected together to move the data and I haven’t planned to buy 2.5 Gbps switch yet (I have spent money on 10 TB HDDs).

Any clues?