In ZUI v4.0.186 & v4.0.199 volte option is hidden, it might have been intentional, on accessing the volte menu you will see why there is an option to enable volte on both sim and interresting enough controls used, allow for both sim to have volte at same the time. When I tried using two volte sim didn't get the expected result but did get jio volte on my second sim slot with no signal on the first one. Its very buggy, so used volte as you were using before, on first sim slot.
Just install the app below to access volte menu. Download
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