Hi All,
Ring hasn’t solved this issue yet, but Tassie Bean has, so I’ve copied their post below. Just one addition (also from another user), if you turn off VoLTE for one line, that will also work.
Australian user with dual SIM cards on Telstra. Dual sims were also not working for me, but I fixed it (for me!).
Of course, turning off one sim works but not a particularly useful solution. On an iPhone, “Settings/Mobile/Mobile data” points to one of the SIM cards. Then for the other SIM card (the one NOT being used for mobile data), set “Voice & data” to be 3G (with your Sim being used for the data no doubt set for 4G). Then it works for me! Both sims are happily able to send and receive phone calls and texts, while the cellular data is taken from the nominated one.
In my case, if I set “Voice & data” for both sims to be 4G the main ring app doesn’t work (I also agree that the Rapid Ring app DOES work in this case, but I foolishly deleted it, and can’t reinstall again).
So, give that a try! There doesn’t seem to be any downside with setting the non data sim to be 3G (until your carrier turns off 3G, but that’s a few years off yet, and I would think Ring will have got their act together by then!).