router which supports 3g dongle

Solution
Asus AC2400 RT-AC87U Dual-band Wireless Gigabit Router

http://www.cnet.com/products/asus-ac2400-rt-ac87u-dual-band-wireless-gigabit-router/

Was just watching the review earlier, and reportedly you can put a 3g of 4g dongle into one of its 2 USB ports. Serious piece of hardware with a $280 price tag to match.
Asus AC2400 RT-AC87U Dual-band Wireless Gigabit Router

http://www.cnet.com/products/asus-ac2400-rt-ac87u-dual-band-wireless-gigabit-router/

Was just watching the review earlier, and reportedly you can put a 3g of 4g dongle into one of its 2 USB ports. Serious piece of hardware with a $280 price tag to match.
 
Solution
This is a feature in the latest firmware release from asus. It will run on many of their routers that support USB. Even some of the older ones will now do it with the latest firmware. Many of the TP-link routers also do it. The largest support is likely on a dd-wrt firmload loaded into a router that has USB ports...again the asus and tplink device. A manufacture called cradlepoint is the oldest one to be doing this and tend to have more support.

The big "BUT" here is 3g/4g/hspa/lte etc etc mean nothing really when you talk support in a router. The routers need to support the particular dongle from a manufacture and then they also must support the ISP you use. Besides the frequencies used for data being different some ISP intentionally rig it so their device do not work on these routers. They want to force you to buy their router.

So what you need to do is first find a router that supports these dongles in general and then very carefully read the list of supported devices. You must match the model number and the ISP exactly or you can not be sure it will work.