Remove both One SLs as surrounds from the Arc in the Sonos app. Unplug the Arc and Ones SLs from power for a couple of minutes. Reboot your router and phone. Swap the positions of the two One SLs and plug the Arc and One SLs back in. Wait until they connect to your network and make sure all speakers have WiFi enabled in the Sonos app. Add the One SLs as surrounds to the Arc.

Some suggestions to perhaps consider…

• If you have wired the Arc and disabled its WiFi adapter, then you need to go back and enable its wireless as it’s almost always required to communicate directly with the Surrounds/Sub.
• Make sure that the surrounds and Sub are not wired to the network.
• If any Sonos devices at all in the system are wired to the local network, then ensure the SonosNet and 2.4Ghz WiFi channels are set at least 5 channels apart. Ensure the first wired product (root bridge) is linked to the primary router only, but positioned at least one metre away from that router.
• If no Sonos devices are wired, try rebooting the Arc and if that doesn’t solve the issue, try changing the routers 2.4Ghz channel to a non-overlapping fixed channel 1, 6 or 11. If the router allows, also set the bands channel-width to 20MHz only.

Thanks for your tips and suggestion. I’ll try them in the next few days. 

is 2.4Ghz better than 5Ghz? ASUS RT-AC88U is in the same room as the Sonos products. On the SONOS app I did see both 2.4 and 5 networks available. 

In simple terms the 2.4Ghz band is much better at penetrating through walls, has a longer range, but is slower… The 5Ghz band covers a shorter distance, but also provides the fastest speeds.