Redmi Router AC2100 slow lan

Dear All,
I have 2 Redmi Router AC2100 routers on my desk, both connected to my all Gigabit home network through their WAN ports. I can only measure LAN speeds of 100Mbps (as measured by on a server on my LAN), even though they are supposed to have Gigabit ports.

Both routers are new, both updated to the latest OpenWrt build (21.02.3)

Any idea why and how I can bring them up to a better speed?

Thanks for any advice in advance!