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How to setup two interfaces on one NAS to isolate WAN and maximize LAN traffic speed?

How I should setup my NAS network settings to isolate WAN and LAN traffic on each physical interfaces (1 and 2) to maximize LAN traffic speed? Or it exists another option to achieve this result without make changes on LAN clients?

I have next home network setup – router with WAN port to ISP and some LAN ports, linux-based NAS with two ports and some PC.

      ┌────────┐
      │  ISP   │
      └─┬──────┘
        │
        │ WAN port
      ┌─┴──────┐
      │ Router │
      └┬┬──────┴┐
       ││LAN ports
       ││       │
     1 ││2      │
┌──────┴┴┐      ├──────┐
│  NAS   │      │  PC  │
└────────┘      └──────┘       

My router setup out-of-box – DHCP for LAN and dynamic IP for LAN connections. And its relativity old – without link aggregation (IEEE 802.3ad) support.