I use https://www.tradingview.com/ and the alerting function they have built into their platform. It's a freemium service which limits to a single alert on the free plan. The mobile app is very good and covers many markets in addition to crypto.
Desktop-only solutions include https://cryptowat.ch that has alerting built-in and is very slick.
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There are also other alerts that are useful like whale alerts etc. But for these you would need to go beyond exchange apps and seek services on the internet that provide them.