Walang automatic way using the default Modem/Router ni ISP.
best way mo without spending money is like yung sinasabi nila na whitelist, hahanapin mo lang kung alin doon yung devices mo, bali kung anong device lang yung nasa whitelist, yun lang makaka connect.
If gusto mo yung automatic, you can try yung voucher authentication, yung mga ginagamit sa piso wifi, cafes, SM. wherein may countdown while browsing, pag nag 0 yun, automatic disconnected yung device, need to provide another voucher para mag replenish.