Started having this issue with my iPad Pro and iPhone 7 plus.
I'm a network security engineer and travel for a living as I work for a consulting firm.
found this thread tried the suggested turn off block pop ups and other settings in Safari that got me thinking its not the browser and changing those wouldn't really do anything more than what a simple power cycle of the device would the problem was with the network settings at the os level not the safari application.
the fix is simple.
settings > general > reset > reset network settings
you'll have to to enter your password and the device will reboot.
You should be able to connect to hotel and any other wifi hotspot that requires authentication outside of the normal password methods ie: a pop up box that ask to confir your name and room number to verify you are a guest at the hotel.