I understand that your network is currently configured as cable_modem~SPA2100~WRT160Nv2~computer.
If you suspect the SPA-2100 of causing issues with some computer application programs then I'd suggest changing the device order to cable_modem~WRT160Nv2-computer and confirm that the computer issues have been resolved. If that solves the problems then you can leave it that way and connect the SPA2100's WAN port to one of the four switch ports on the WRT160Nv2 router.
To find the IP address of the SPA2100's WAN port use IVR code 110. In order to access the menu from the WAN port you can use code 7932 from the phone handset as explained
HERE.
If you experience problems when operating the SPA2100 behind the other router, try the following settings:
(under SIP tab)
Handle_VIA_received: yes
Handle_VIA_rport: yes
Insert_VIA_received: yes
Insert_VIA_rport: yes
Substitute_VIA_Addr: yes
Send_Resp_To_Src_Port: yes
STUN_Enable: yes
STUN_Test_Enable: yes
STUN_Server: stun.voxalot.com.au:3478 (or any STUN server such as 'stun01.sipphone.com:3478' or 'stun.sipgate.net:10000')
NAT_Keep_Alive_Intvl: 179
(under Line_1 and Line_2 (or PSTN_Line) tabs)
NAT_Mapping_Enable: yes
NAT_Keep_Alive_Enable: yes
NAT_Keep_Alive_Msg: (this should be blank in order to send small packets that do not cause proxy to reply)
NAT_Keep_Alive_Dest: $PROXY