GST under reverse charge on office rent

Does monthly office lease rent of ₹ 1.5 lac i.e. ₹ 5000 per day paid to unregistered lessor falls in this category? if not, does GST under RCM payable by lessee on this? the lessor is supposed to get registered (rent being more than 20 lac p.a.) and charge GST on rent but if he is not charging it why does it become lessee's duty to charge GST under RCM. Also how does one issue Self Invoice and Payment Voucher for GST paid under RCM, could anyone please clarify? thanks!