Since you don't want to use it at "home", don't set it up with your home wifi network.
If your phone supports being a wifi Hotspot, do this.
1. Set up your phone's Hotspot. The pictures are from my Android phone.
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Be sure to choose the 2.4GHz band.
2. Open the Ewelink app, click the plus sign at the top to add a device, and click quick pairing.
Fill in the network name and password from step 1, make sure you phone Hotspot is enabled, and follow the instructions from here.
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After this is done, you will be able to use the Sonoff device any time your phone Hotspot is on and within range.
If your phone does not have the Hotspot feature, you're out of luck if you want to use the Sonoff away from whatever network it's configured for.