SFP port configured in Trunk mode not servicing tagged VLAN traffic to other branded switches

We have replaced our Cisco switches with Netgear M4300s. We currently have an active AWS Direct Connection provided to us, which was working just fine while connected to a Cisco 2900G. The Cisco port was configured as a trunk, allowing vlan 40.

The hand off is given to us via Single Mode fiber, which we have connected using a SFP 1000Base-LX module. We have layer 1 connectivity (link lights are active) which has been confirmed by AWS Support - they see a good light signal on their end. This is connected to port 12 of our
M4300-12X12F. The port's configuration is as follows:
interface 1/0/12
speed 1000 full-duplex
switchport mode trunk
switchport trunk allowed vlan 40
exit

We have concluded the issue is with the layer 2 switching between our switch and the providers (which is unknown and not in our control). We are not seeing any mac address on the other side of this connection, only our router which is configured on the same VLAN.

We have other ports configured in trunk mode, servicing other VLANs on our network, which work just fine. We have attempted to use other SFP ports on both of our 12X12F switches, with the same result.

Our firmware version is:
12.0.11.18
Which is the same across our stack of 4 switches (2 x M4300-12X12F and 2 x M4300-52G) connected via spine and leaf.

Any help is appreciated.

Thanks a lot.