Go to device Settings > General > Profile & Device Management. Tap on the prompt to Trust the app from Synergy Financial Advisers Pte Ltd.
For Android 8.0: Before downloading the app. Go to device settings > Apps & Notification > Advanced > Special App access > Install Unknown Apps > Select your browser/application that's used to download the app > Toggle on allow from this source.
For Android 7.0 and below: Before downloading the app. Go to device settings > Security > Toggle on unknown sources.