Someone else should comment... but my pal has the M5 Deco 3-pack. He is on Plusnet (I think) so cant do modem mode at all with their Hub. So he just plugged the first Deco into one of the Plusnet router ports on an ethernet cable, registered it on the TP-Link website and then connected it all up on wifi to the other units via the App. He said it was the simplest thing he has ever done and it all works perfectly. I think its the "default" connection mode for the Decos.
So his Hub is still in normal router mode.
EDIT: Just remembered.... another Pal on VM did the same set up and its working fine.
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John
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I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired, 1 WiFi, 1 on PA) Hub5 in modem mode with Apple Airport Extreme Router +2 Airport Express's. On 250Mbps, Talk Anytime Phone, x2 Mobile SIM only iPhones.