Hi,
Add "delay = yes", without quotes, globally or per-service (on client stunnel). It's enough.
You only need to have in mind that the dynamic DNS provider do its best but all depends on the whole DNS network, so, even setting this on stunnel client service, are needed about 1, 2, 3 minutes, or even more, to propagate the new IP and with the luck that DNS providers don't cache the for long times the resolved names, if you don't use your own DNS server.
Regards.
P.S.: the time I said is, more or less, what No-IP "lags" to propagate, according to my experience with it.