It seems Marriott's outsourced WiFi provider (BlueprintRF) doesn't approve of people using travel routers and are blocking them.

I first hit this after evacuating from a hurricane last year, only to get to the hotel with my work computer AND work travel VPN router, only to have it blocked. I called the 800# for tech support and they confirmed they were blocking routers and weren't budging. Great. I've evacuated because of a hurricane, trying to keep our business going, keeping in touch with people still in the affected area, and coordinating emergency relief and emergency staffing...but Marriott decided that was a "security threat", according to BlueprintRF's tech support rep. The rep said only the hotel owner or GM would be able to allow this to happen. Fortunately, the front desk agent was more than happy to um.."have the manager"...approve. The tech grumbled, telling him this was a "security threat to the entire hotel."

My router is one of the first things I unpack at a hotel, and I've stayed at ~10 other hotels since with no issues... until this weekend it happened again, this time at a TownePlace. Even using cloned Apple MAC addresses wasn't helping. I wasn't planning on working this weekend, so I didn't bother. BUT, some arse elsewhere in the hotel kept trying to cast videos to my streaming box, so I guess Marriott doesn't enforce isolation between devices on their networks. So much for "security".