That’s what I would suggest as a starting point. Can the OP confirm by testing that 3G calling works in his area? (That is: phone is showing 4G, commence a call, phone drops down to 3G, call works, when call ends phone goes back up to 4G.)
If the OP has not already enabled VoLTE then I would have to assume that 3G calling is working just fine (otherwise all calls should be failing and the OP would have noticed this).
If 3G calling is working just fine, based on the published schedule, enabling VoLTE should still be on the OP’s radar for some time this year but it may actually be unhelpful to enable it now.
I say this in the sense of … it’s a potentially unrelated change without theoretical justification and is just moving the goal posts and potentially introducing other variables.
The opposite perspective is … we don’t know what is causing the problem with DTMF so we might as well try something (as long as the OP knows how to back out i.e. disable VoLTE if things get worse not better).
I went looking to see whether there are any notable implementation differences between out-of-band DTMF over 3G and out-of-band DTMF over 4G, but didn’t find anything.