More information needed:
Your boss asked if one of “their accounting softwares could be connected…”. Who is/are “they”? If you are asking if you can create a connection to a client – server based accounting system using a DSL line as the physical WAN link, yes you can. The question then becomes how do you allow access (either from “them” to you or from you to “them”) from the client network to the target network?
What OS are you using? If Windows, there are several ways this can be done, including creation of an AD account for guest access and creation of a VPN account (which might also require set up of a VPN access server or access point), or a simple RDP (or Dameware or VNC or…) session into the server .
If you are using a Mac based system, SSH should work, as well as a number of other UNIX/Linux methods with which I’m not entirely familiar.
OS/400 (iOS)??? No knowledge….