@DeathSwitch For me the problem was a faulty bridge mode on the modem... IDK if you're in the same situation, but I thought it could be useful to share. Reverted to access point mode with the decos.

@Alexandre. Got what I needed by switching to access point mode. A bit sad to lose all the router functionality of the decos, but that will do for now, at least I don't have any double NAT problems...

Well it does now... My ISP lists an IP and MAC adress for the deco that is not the one in the TPlink app (1 digit different)... So I'm having trouble forwarding a port from the modem to the deco to...

Maybe I can use a DMZ on the modem in router mode instead ?

@TP-Link Thanks. Indeed those answers were helpful, but It still does not work with modem on bridge mode (see post update). Same as OP here.

Update : As I feared, putting the modem back in bridge mode brings everything down. VERY unstable wifi and cable connection with DNS errors. Cannot connect to decos using the app, even with reboots...