Can you explain a little further about your network to help us understand your question?
What other network equipment do you have? Is there a Router to the Internet? Is the AP connected to a Switch?
Are you trying to create a Local Area Network with no Internet connection, just the AP and things you want to connect?
An R310 only has one Eth port, so you cannot route a new LAN network NAT translated, thru the AP to your 10.20.1.x network.
If you had a 2 port AP, the 2nd port LAN would need to use a VLAN matching the Local network you define.
You could route the AP's wireless clients, NAT translated from a new local network, thru the AP to your 10.20.1.x network.
As all these clients would be wireless, you could use any VLAN on the Local network, without a router for that VLAN (and
assuming you only intend to use this one R310 AP).