Some thoughts of mine (I know nothing about BT ONT configuration): Perhaps you are facing a speed/duplex mismatch.  I'd try locking the config at 1000/full for a test. DHCP has a 1500 byte MTU.  PPPoE has a 1492 byte MTU.  Perhaps you are experiencing an MTU squeeze.  You can test this on windows by running: netsh interface ipv4 show subinterface  Note the name of the interface being used for Internet traffic (it will have lots of traffic on it). Then temporarily change the MTU to something like 1300. netsh interface ipv4 set subinterface “Local Area Connection 2” mtu=1300 If that resolves it, you'll need to request Meraki support to lower the MTU for you. Perhaps BT use a VLAN tag for the Internet service, and you are falling into some default service with lower performance by not using the same VLAN tag. ... View more

So is the port on the MX set as a trunk allowing VLAN 2? And the same for the 9200 side? Typically I would expect an access port on the switch to be set for VLAN 2. The link between the switch and the MX would be a trunk and allowing VLAN 2. I wouldn't typically accomplish this using group policies. ... View more