Xiaomi Mi 3G slow Wan speed


I have a Netgear Wndr3700, running an old version of LEDE.
It's connected to my ISP via another box in bridge mode (French Freebox v5)
WAN speed (measured from a computer in the LAN) is about 14Mb/s

I'm trying to replace it with a Xiaomi Mi 3G (OpenWrt 19.07.3)
Unfortunately, WAN speed get very slow (~4Mb/s) when I do so.

The curious point is that, if I connect my computer to the Mi 3G and the Mi 3G to the Wndr3700 (with different lans for each router), speeds are good again.

To sum up :
Computer->Wndr3700->ISP : 14Mb/s :slight_smile:
Computer->Mi3G->ISP : 4Mb/s :frowning:
Computer->Mi3G->Wndr3700->ISP : 14Mb/s :thinking:

I suspect some mis-negotiation of a parameter between the Mi3G and my ISP's box, but I don't see which one (output from ip link and ip address are almost identical between the two routers)

Any ideas ? Any parameter I could check to spot the difference between the two routers ?

Thanks !

edit: forgot to mention, everything is done on rj45, wifi is not involved here...

Not sure about the policy about that here, but trying to bring back this thread, in case anyone has any idea.
Cheers !