Self assessment and "taxable" capital gains

I moved to the UK from the US in August 2021. I have been employed on a PAYE scheme so have paid my income tax in the UK that way, but I have retained some investment accounts in the US. I paid tax on these accounts when I filed my US tax return, but my it's been my understanding that I still need to register for Self Assessment to report that income to HMRC. I haven't been able to register for Self Assessment online so was filling out the "paper" form to register, but I wasn't sure what reason to select for why I require Self Assessment -- is it because I have taxable foreign dividends or capital gains tax to pay--or both? I earned ~$4500 in total dividends and ~$15k in capital gains across these accounts over the US tax year. (And if it is because of taxable foreign dividends, what date do I use for when I started receiving dividends? I've had these accounts for much of my life.) Please let me know if I can clarify further -- thank you!

Hi
It will be both.
Use  the date you came to the UK as the start date.
Thankyou.
Regards.