My attribute table (as in the attached screenshot) shows that there is only 1 element and the rest of the 26 items is filtered out. How can I turn off this filter?
There is the Query Filter which is defined directly on the layer. It is a global filter. It filters the data at the moment it enters QGIS and therefore applies to everything, including the attribute table.
Might potentially be just the view you are using, if you see bottom righthand corner, change view to table view, or de-select the element that is highlighted? Maybe helps?