Extremely slow speeds between company VPN and 814-v6 Residential Gateway
I've been provided with an Adtran 814-v6 Residential Gateway by my ISP (Hey! Broadband).
All of my personal devices are able to achieve ~99Mbps DL and ~50Mbps UL via the 814-v6 Residential Gateway, which is great...
... however, my company laptop uses the Cisco AnyConnect Secure Mobility Client to connect to my company's VPN, and it is with this connection that I'm observing extremely poor relative performance of 0.5Mbps DL, and 5Mbps UL. As you can imagine with these speeds... it's very difficult to even hold a video call whilst working from home. Whilst I understand that the DL traffic through the VPN is expected to be reduced, going from 99Mbps DL down to 0.5Mbps DL smells particularly fishy...
Note that Cisco's VPN client uses IPSec, and so I had to open ports 500/udp and 4500/udp via the "NAT -- Port Triggering" menu within of the 814-v6 Gateways configuration pages. This is described in the user manual "Configuring the 814-v6 Gateway.pdf". After enabling these ports, I can observe via Wireshark that the ISAKMP traffic is able to use ports 500 and 4500 to allow my Cisco Annyconnect Client to establish a connection with my company's VPN.
However, I'm just not able to figure out what's throttling the speed between my Cisco VPN client, and the 814-v6 Residential Gateway. I have spoken to my ISP about this, as well as my company's IT team, but we have not yet arrived at a resolution.
Note that I have previously connected to my company's VPN using a 4G MiFi router (typically achieving ~8Mbps UL / 15Mbps DL), and other routers provided by other ISPs (BT, Sky, etc), and have not experienced this sort of throttling of my connection before. For this reason, I believe that the issue resides squarely within the configuration of the 814-v6 Residential Gateway.
Any ideas what the issue could be? And which configuration tweaks I could apply to resolve the issue?
P.S. please be kind and assume that I have very basic networking knowledge, and apologies for the long post :).