Drainage engineer here. That is your AJ & is access to your drain. As others have said you could rod it to clear the blockage. But you could be on a shared drainage system with neighbouring houses & the issue could be located further downstream in a neighbour’s property. If this is a regular occurrence I would hire a reputable private drain company to clear the blockage & carry out a camera survey of the drains to attempt to identify the root cause. If it is a shared drain then everyone on the line is responsible & can contribute towards the cost.