@AndrewBL,

I would have to agree here, something is not quite making sense.

Are you able to check the speeds of these devices with a wired connection such as a PC? Ethernet Speed tests are truly the only accurate indicator of what each node is receiving, and it is possible that it is only the wireless connections that are being affected.

From your post, though, it sounds like the main router, even alone, does not even get close to your expected speed; it is only the third deco node that is able to receive the whole bandwidth?

I would first double-check your QoS settings to ensure it is not too low. You may try to manually set the QoS equal to or above what you are expecting to receive from your ISP.

Your Ethernet Cable, is it CAT5? or CAT5E? A CAT5 Cable cannot go over 100mbps, but this would not explain your third decos speed.

I would honestly recommend resetting the main node and reconfiguring the network, as it appears as though something is not negotiating the speeds correctly. If you do not want to reset your network, try simply swapping the main node with another node, and changing the main node setting in the deco app. This will cause the network to reconfigure itself appropriately. 

  @bibi9768,

A few things - the wireless speed test app on phones is far less precise than using a wired connection to test the speed. Even if it appears that full speeds are being reached, there is the potential for speed tests to be inaccurate due to the features of the service and server you are connecting to. It is also important to consider that when testing wirelessly, your phone may not currently communicate with the closest node, or may be using the slower 2.4GHz band.

If possible, first test the speed with a wired connection to see what speeds the third Deco is receiving.

Normally, the speed being limited to 100Mbps is an indicator of a problem with the cable. Even if your cable is rated to support the speeds, it is possible that a fault has been developed or the cable has been damaged. To test this, move the satellite (third) node into the same room as the main deco and wire the devices together. Then test the speed with a wired connection to see if the node can receive gigabit speeds.

I would also recommend trying to swap Ethernet Ports on the nodes, as all the Ethernet ports have Auto WAN/LAN sensing, so it does not matter which port is used(other than the speed ratings of the ports)

Lastly, while Ethernet Backhauls are preferred by a long shot, try testing your network while using a wireless backhaul to the third node. If there is a problem with the cable, you may see higher speeds.