Lifting the next chamber downstream tells you where the blockage is. If that chamber is empty, the blockage is between the two. If it is also full, keep working downstream until you find an empty one.
If every chamber on your property is full including the last one before the boundary, the blockage may be in the public sewer — in which case your water and sewerage company is responsible and will attend free of charge.
Never pour water or push a hose in to "help it along". You will simply move the flooding to another chamber or into the property.