Once you purchase the Remove Ads in-app purchase in Timer+, ads will no longer show up in Timer+ for all of your devices tied to your Apple account (your iPad or iPhone, for example), and also when you switch devices (getting a new phone). You will be charged just once for the original purchase; all subsequent removals will be free.
To remove ads on a second device, or when you've switched devices, do the following.
For more information, check out Apple's article on In-App Purchases. (Timer+'s remove ads purchase is a Non-consumable type.) support.apple.com/en-us/HT202023