The DE310 phones do not support wireless connectivity so you will need to use an Ethernet cable to connect the phone to the local network - via the router or a switch. 

If it is near a power socket they could look at using something like a HomePlug:  

This is not something we supply and do not recommend for a business in general but they can be used at home and may be a workable solution for you if you do not wish to run a cable.