Can't reach GS748T Web UI outside of local network

Hello community,

I have a GS748Tv5 switch on a particular network, along with a GS724TP. They have separate IP addresses (obviously) on the same network and both have the same (correct) router configured as gateway and the same net mask.

I can manage either switch from within the local network, but if I try to manage the GS748Tv5 switch from an outside address (i.e., one that passes through the gatewal/router), the Web UI won't come up. It looks like it's starting to, and part of the page seems to come across, but not enough ever loads to display anything but a blank page on my screen. I can ping the switch just fine; I just can't manage it through HTTP or HTTPS.

For some context, the "outside" address I am trying to access the switch through is on the other side of a site-to-site VPN established by the router. The other side of the VPN tunnel is my local network (I am an outside IT vendor and I manage this organization's network from my office and local network through the VPN connection to the organization's internal network). I know the VPN tunnel is configured properly, because I can reach everything else on that network (various web interfaces including the GS724TP switch, remote desktops, SMB file sharing, SNMP stuff, even SSH protocol) from here with no problem. But I cannot bring up the web interface of the GS748T without going through a remote desktop session to some computer on the switch's local network.

As far as I can tell, there are no filtering/firewall features in the GS748T that would prevent it from being managed across the VPN tunnel. The only other thing I can think of is that it might be something related to MTU, but there doesn't seem to be any way to set MTU in the GS748.

Any thoughts? I'm running out of ideas here.