I believe MAC AUTH authentication would suit your use-case best: https://wiki.teltonika-networks.com/view/RUT950_Hotspot#MAC_auth
But, this will require some changes and scripts on the template that hotspot is using. If I understood you right, you want one client (let's say one MAC address) to connect first, and when it successfully connects - others are allowed (MAC addresses are added to the MAC filter pool).
For this, there must be a script that recognizes that one device as a special one, and then somehow it must catch that it went through the landing page. Some kind of boolean value maybe?
After that script should initiate the adding of MAC addresses to the filter which are then allowed to go through the landing page too.
What can I offer you in this situation is what we call "GPL packages", which really are source codes of our firmware. In these, you will be able to find the code for generating templates or maybe add a compiler to your preferred language.
If you'll find this really difficult and you see that this functionality is really necessary for you that you even would spend money on this change - please contact your sales representative for MBP.