May Allah bless you.
For stock ISA bought with the intention of capital growth, zakat will be due on its total market value on the zakat anniversary date.
If the stock ISA has been acquired for dividend purposes only, then zakat will be due on 25% of its total market value, and the residual dividend amount on the zakat anniversary date.
I don’t have much details about the ISA offered by Wahed, but @Mufti_Faraz_Adam is their advisor, and can confirm the above.