I am attempting to refinish a bathroom in a storefront I am renovating. I pulled up the cheap vinyl tile to reveal backer board, which was supported by shims over a sunken concrete slab. To make matters worse, the concrete around the toilet flange had been chipped away, leaving a 2'x2' hole into the crawlspace below. Any thoughts on the best way to tackle this mess? this doesn't have to be pretty, just stable. I will be using vinyl tile in the bathroom.

regrout bathroom