Yes, if the contract-holders (tenants) remain in occupation after the end of the fixed term and you haven’t signed them up to a renewal then the occupation contract will continue as a periodic occupation contract on the same terms as the fixed term contract except that other statutory terms will automatically be included in the contract.
The Renting Homes (Wales) Act 2016 requires you to send the contract-holders a further written statement of contract when the contract turns periodic. In order to make things easier for you we have provided the below document which can be sent to the contract-holders during the fixed term and will deal with your obligations. We recommend that you send this to the contract-holders during the fixed term so that you won’t need to send a further written statement of contract when the contract turns periodic.
Of course an alternative easy option is to sign the contract holders up to a renewal tenancy agreement.