Captive portal and self registration by SMS for several million users

[fathibn]

Hi,
I have a particular setup where opnsense would be put on trains to filter and shape internet traffic.
What I am trying to setup is a captive portal with self registration by SMS: user is promted to enter his phone number, upon receiving his password he can connect with his phone number as username and the randomly generated and sent by sms password.
Also, this authentication system should be centralized, meaning when a user registers onboard of train A, leaves it and rides train B, he hasn't to reregister and should be able to login with credentials created on train A.

I am thinking of a centralised auth system, like a central radius server but would also like to minimize traffic between each embedded opnsense and the central auth server (no accounting for now).

Does anyone have a similar setup ? How did you solve thess issues ?

TIA
Fathi Ben Nasr