Take a look at this thread:
http://mybroadband.co.za/vb/showthread.php/400897-MikroTik-RouterBoard-and-USB-3G-Modems
Lots of useful info there regarding that particular model of MikroTik router.
I have put a lot of work into various scripts and configs for this.
I have just actually set up a RB751U-HnD as a basic hotspot using a 3G modem plugged into the USB port for the wan connection.
Unfortunately the hotspot wizard feature of this particular model does not work straight from factory defaults and results in a "broken" hotspot. You need to delete the default Bridge (bridge-local) first to get it to work.
First reset the router to default then delete (Remove) the existing Bridge config and give the "Hotspot Setup" a try again.
Oh..and do upgrade to RouterOS v5.19
I have a few of these routers running this latest version, with no problems.