There seem to be several different topics here.
The first question seems to be asking for a way to access settings when the gateway has only an LTE connection. If you are actually right next to the box while it is on LTE, running its internal wifi in access point mode would work for local access to the settings, but not for remote administration.
For remote administration, generally speaking mobile networks do not allow inbound traffic (though there are sometimes special offerings). So what is usually done is to have a daemon make an outbound connection to a cloud server which forwards a port back. “ngrok” is a commercial offering of such a service but might not literally work in this case. Using openwrt scripting I have a solution mostly working. I don’t think this is a stock feature, though it would be a good one - and of course it needs a cloud server to function at the “jump host”.
The next major question is if the RAK7258 in the newest question is actually the LTE model. All models have the SIM slot, but only some have the LTE modem soldered to the board underneath the LoRa card. If it isn’t an LTE model, it is possible to connect an external LTE card to the USB port, but I don’t know if the stock software would support this. We needed a very specific LTE card so have gotten this working by rebuilding openwrt with chat as well as ppp and copying over the chat & ppp scripts we worked out on a larger system in order to run it.
A simpler solution could be to connect the gateway to a mobile access point by Ethernet.
For a portable demo, the simplest of all may be to tether it via wifi to your phone.
(Beware that some hotels and conference venues go to efforts (often illegally) to block the use of the personal access points, in order to charge high fees for access to their in-house network. Where this is done by attacking foreign wifi networks, an Ethernet linked AP would be workaround)