Tom, try to configure WA901ND in access point mode. Its SSID and password can be changed to be the same as the main router. Remember to use different channel for main router and WA901ND.

Very old model. When did you buy it? You should update your network equipment.

Since port forwarding works when firewall is disabled, I don't think configuration on virtual server is wrong. Why you have to enable firewall? What are you trying to control? After you configure it...

What's the model number of your printer?

Cannot work together.

Is there a modem or cable connection to LAN4/WAN port of D2? D2 has been configured in wireless router mode? If so, PCs can only plugged into LAN1, LAN2 and LAN2 ports to access internet and IP...