The router WAN connection is most likely an 8-pin RJ-45 ethernet connection. It is not a telephone connection. This is typical of routers.
The BT Voyager 100 is a simple modem with only a USB connection. It is intended to plug directly into a USB port on a single computer. It has no ethernet port - so it can't connect to the router.
If you want to keep the router, try to get another modem that has an ethernet port. Then plug the new modem ethernet into the WAN port on the router.
or - you might use the hardware you already have as follows:
If you want to keep the modem here is one relatively inexpensive way to connect more computers: This assumes that all the computers have ethernet ports (8-pin RJ-45 as above) Assuming the operating system on one of the computers supports Internet Connection Sharing (ICS), do this:
Plug the modem USB into Computer #1 Enable ICS on Computer #1. Plug the router WAN connection into the ethernet port of Computer #1. Plug the other things into the router ethernet ports. Plug the printer into the router USB port .... that might still work OK.
This may require some software configuration on Computer #1 - but it should work. Computer #1 will have to be turned on in order for the others to reach the internet. Have a firewall running on Computer#1.