You can use Trading View's built-in trading view alert option, it has a bundle of options for web-based usage and it will also work on smartphones if you download the Trading View application. And the amazing thing is you can use Cryptohopper platform that generates buying and selling action API that you can add to your Trading view or any other alert app. You can use Coingecko or coinmarket cap too but the experience is not good with both and the fun fact is they both show you the average price of the market, i.e. BTC.

Therefore you might not get a good deal, if you are planning to trade only one one or two exchanges. The best practice will be to use a built-in alert system of exchanges that you are going to use for making trades because each exchange does have small differences between the prices and the average prices from coinmarket cap and coin gecko might no be of any benefit to you.
There are bundle of alert apps but they don't work efficiently, I suggest you to use telegram bots like https://cryptocurrencyalerting.com/bot/telegram