At the end of a certain tax year (5th April) and after the new one begins (6th April), the tax-free allowance of £20,000 will be reset automatically in your Trading 212 ISA. You'll not need to open a new Trading 212 ISA, as you can accept the ISA declaration and add funds for the new financial year directly in your existing ISA with us.
If you decide not to contribute to the new tax year in the Trading 212 ISA, you should make no further deposits. You can keep your holdings and reinvest the contributions from previous tax years.