I'd like to clarify a little bit: Your "Uni-Fi" is referring to Ubiquiti Access Points (I love their UAP LR, best thing imho), some people in the USA seem to have an ISP called UniFy or something like that, so there is slight potential of confusion.
On the UniFi Controller Settings following is present in Restricted Subnets. However, there is nothing in the allowed subnets.
These default subnets are the ones considered as being private IPs inside the UniFi Controller. Let's say you have a guest WiFi for the Hotel, people cannot access these subnets.
(Temporary, to resolve the issue I had to plug in my old router back to the network and everything is working OK with old router and no issue with DHCP or UniFi connected devices. )
This fact states that your RB config seems the problem. Let me set up an RB951 and send you the config which I think should be functional, I will be able to do this in ~5 hours.
Info: The 169.254.x.x indicates APIPA Operation, so the Windows Computers cannot reach any DHCP server, the second hint to take a closer look at the RB.
Regarding the question if the RB951 is suitable for ~50 people I am sure it is, at least in the basic config (no fancy QoS, etc). How much bandwith do have at WAN side?
When sizing a a router, effective measures are:
- How many people max at the same time (Number 1-100000)
- bandwidth demand (MBit 1-1000)
- VPN required? (yes/no, if yes, how many concurrent)
- Features like QoS? (yes/no, how many expected services/layers)
All those decisions boil down to how much WAN2LAN Throughput the device must be capable of pushing AND if the CPU and RAM must be powerful.
Taking a quick peek at the engenius device states 500MHz CPU and 64MB RAM which seems great. Since the RB951 has comparable specs you should be fine.
I like the Winbox usage instead of the WebFig since Winbox is faster but that's just a matter of taste.