I recommend that it is simplest to use EUR. 1. That will make money transfer easier and 2. you will more easily have the correct tax records, since dividends and the proceeds of sales will be converted into the correct number of euros on which you will eventually need to figure your tax bill. If you try to run a USD account you will have to make a note of the exchange rate to use for tax reporting, every time a dividend is paid and every time you buy or sell stock. Since Trading 212 charges no currency exchange fee, there is little to be gained by keeping your account in USD. Multicurrency accounts are desired by many - but using them will require substantial record keeping of day-by-day exchange rates in order to figure the correct tax in local currency.
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