Reinvesting your dividends is made easy in the AJ Bell app. Just go to your 'More' menu and then 'investing' and select 'Dividend reinvestment' From there you can choose the specific investments you want to have the dividends reinvested for, or if you wish, select for them all to be reinvested.
Because of the handy on/off slider, you can change your reinvestment selections anytime.