Try these steps:
1. Connect pcWRT WAN port to modem, disable WiFi in the Wireless Settings page.
2. Connect the Linksys 1900AC WAN port to a pcWRT LAN port.
Follow the steps below to set up the Linksys AC1900 as an access point. In the paragraph below, new router refers to Linksys, old router refers to pcWRT.

To test the connection, connect a PC to a LAN port on the Linksys 1900AC. Can you browse the net? If not, check the IP address of the PC: click Start button, Run, type cmd, then click OK. In the command prompt window, type ipconfig then press Enter. Check the IPv4 address. It should be in the 192.168.10.x range.
If you can successfully connect to the Internet, repeat the same test via WiFi connection to the Linksys AC1900.