Have you tried to use Tradingview?
I just recently checked that you can also set an alert/alarm for volume too, like if the volume cross at particular number.
Tradingview Appstore:
hereDisadvantage of tradingview is you can't set or create unlimited alerts if your account is not premium, so you will pay for premium subscription.
And also, have you tried too the
TabTrader, they have an IOS app too. Since I tried this app long time ago on android, you can also set an alarm or alert on prices, but I don't think you can also do it on volume.