After much monkeying around, I think I've fooled my Jetpack -- at least for the time being -- into acknowledging the fixed IP address of the T2. I did this by first enabling DHCP on the T2, getting an IP from the Jetpack, then disabling DHCP and using the address the Jetpack had previously assigned as the fixed IP. The result is that when my T2 connects to the Jetpack, the Jetpack shows it with the correct IP address. I don't know if this is a workable solution long-term, but as a troubleshooting step I think it moved the ball downfield some.
But I still can't get corrections, so now I'm wondering if my LS is setup properly. What are the Username and Password fields in the RTN TCP Client screen? I don't think I've changed these (I redacted my password for the purpose of posting), but I don't know where these values are needed.
Also, since I've been getting way farther into the networking weeds than I ever wanted to get, I'm wondering why a DMZ and VPN passthrough are needed at all. From what I've read, enabling a DMZ allows all incoming traffic through to the specified IP, while port forwarding directs only traffic on the specified ports to the specified address. Port forwarding seems like a more restrictive way of allowing traffic through the firewall, and with port forwarding enabled you shouldn't need a DMZ.
I'm even less clear about the VPN passthrough, but my sense is that it's even more secure than either a DMZ or port forwarding, but that it needs a properly-configured device/system on the other end to work. I don't think the T2 is involved in the VPN aspect at all, but again I'm way over my head in this matter.