Here is the short of it..
As the RouterOS box acts like a proxy (NAT) for all devices behind it... when ANY device behind it loggs into the hotspot on the WAN side of the RouterOS box, it appears to the hotspot that the RouterOS box has logged in...
Like wise any traffic crossing the RouterOS device looks like it is comming "FROM" the RouterOS box. (as seen by the hotspot)
So any keep alive type pakets from any device behind the RouterOS box will keep the RouterOS box's connection "open"..
I use this scheem often for trade shows when I get a single login for x$$ and it is good for 1MAC address..
I place the RouterOS box on line and use my management PC to open the conncetion, then log the rest of the users on to the hotspot I run on the LOCAL RouterOS device... can save $100s of for each event..
Usualy the venue does not care.. it is offered as a service.. but their billing system hands out 1 login per MAC and their accountants are greedy...
But you DO need to get this "use" cleared with the hotspot owner or you could find yourself in a "Hot Spot !!"