Hello Morset,
NAT stands for Network Address Translation.
If you plug router A (WAN) into your modem, then router B’s wan port into router A’s switch port (LAN) then every device connected to router B will be double NAT’ed.
INTERNET < ROUTER A (192.168.0.x) < ROUTER B (10.0.0.X) Thus creating 10.0.0.X IP to be translated to 192.168.0.X before it hits the internet. This can cause latency issues definitely with VOIP audio. If you simply need more ports, you definitely need a switch.