I have a foreign (US) robo-advisor investment account that holds both reporting and non-reporting funds. The account will automatically rebalance holdings and reinvest any dividends. The capital gains from any rebalancing is minimal - my gross capital gains and net gains/loss are under £300 and (~£300) in total respectively over the last 5 years, and my total holdings are under the 4x Allowance.
I have not registered for self assessment previously and did not realize I might have to (I have not previously met any other rules that would require self assessment). I wanted to understand what my reporting requirements are and how to remediate.