I have this RB3011 and (AC) wAP and running CapsMan.

I can connect the Brother Printer Wireless, but sometime I can not print from my MacBook or iPhone.
My wife can not even add the printer in settings (MacBook) I have enabled "Client to Client" in "Datapath"

If I scan my network I can see the printers IP (static)

Do I need to enable some UPnP?
Any ideas?

I think the time of lease dhcp change between some time that mean the ip address of printer change.
You can increase the time or applying static dhcp printer

I have this RB3011 and (AC) wAP and running CapsMan.

I can connect the Brother Printer Wireless, but sometime I can not print from my MacBook or iPhone.
My wife can not even add the printer in settings (MacBook) I have enabled "Client to Client" in "Datapath"

If I scan my network I can see the printers IP (static)

Do I need to enable some UPnP?
Any ideas?

I wouldn't enable UPnP.

First off, can your wife's Macbook ping the printer's IP address? Are the proper drivers installed on her MacBook? Can you confirm that the printer is "awake" and not in hibernation when you are trying the initial configuration?