new to pool care

Hello I just made a trip to a pool store as I didn’t find a good chlorine elsewhere. They offered me free pool test and the result came out that my phosphate is way high above the standard range. They asked me to use a phosphate remover chemical they sell, but I wanted to first check in this...

I have a DE filter that looks more like a cartridge (Pentair ECDE60). Does anyone know how frequent the DE should be backwashed/filter grids cleaned? Everything I read says when the pressure gauge raises 10 PSI above normal, but my gauge is broken. My filter is so BASIC it doesn't have a...