Yes, So since you can’t select which network the Rachio can join (even though we should be able to as Rachio says we can) and The Rachio Joins the 2.4 GHz network by default, Can you not just forget the 5GHz network on your iPhone and let it join your 2.4GHz network all the time - as a work-around - for now.
*FYI - If you use iCloud sync services the wifi networks your apple devices join are “remembered” and “synced” to other apple devices so as not to require you to put in password all the time - This could cause your iPhone to rejoin the 5GHz network…)