This is a slightly confusing thing to setup.
The Draytek 2860 (and later similar models) allows for a 3G (or more usefully now 4G) modem to be plugged in to one of the USB connections as an WAN (Internet) option.
There are some instrctions for a similar unit at:
https://www.draytek.co.uk/support/guides/2860-alcatel-l800-setup
These instructions also work well for the EE Osprey (original) which is based on the Alcatel Link Y855.
The Draytek also allows for a mobile phone hotspot to be the WAN2 connection but this may be less useful if WAN2 is the (or one) main WAN connections.
