Are any of your speakers wired to your router or network with an ethernet cable?

Nope they’re all wireless. The arc is about 2 feet from the router and the two surround One SL’s are probably 12 feet from the router. 

Have you tried unplugging both of your One SLs from power for about a minute or rebooting your router? Or disconnecting your One SLs as surrounds and re-connecting them? If possible, you could also try wiring your Arc to the router.

Note, when in use as surrounds, the Sonos Ones are connecting to a wireless signal from your Arc, and not the router.