Check the rent-increase notice against your tenancy agreement and the current RTB rules on rent setting, including any rules applying to your property. Keep the notice, previous rent records and all messages, then ask the landlord in writing to explain or correct the increase. If the issue is not resolved, you can apply to the RTB for dispute resolution, usually through mediation or adjudication, and should follow its time limits and evidence requirements. Continue paying the rent properly due while the dispute is considered, and consider advice from a solicitor or Threshold if the circumstances are complex.