I cant see the pipes returning to the pool in great detail but it does not look like there is a valve there that can be opened or closed. This would leave two main areas that could be causing the high pressure in the filter and the lower return pressure to the pool, these would be the filter itself or the a blockage in the pipes returning to the pool.
What I recommend doing is once you have the pump up and running again (most likely needs to be primed with a hose for a few minutes), wait for the pressure to rise again and feel the water returning to the pool. Then turn off the pump and move your multi-port valve to recirculate. This will bypass the filter entirely and just move water through the pipes. Feel the water returning to the pool again to see if there is a difference, and again after a few minutes to confirm that there is no change.
If it feels like you have good pressure returning to the pool now then the issue is with the filter and we can begin troubleshooting.
One of the most common issues with increased pressure at the filter is dirty grids or way to much DE. The grids are made of cloth and the DE forms a layer over the cloth to filter the water. The cloth can also trap dirt in the fabric which will not allow water through and as a result pressure can build up. To resolve you would actually need to clean the grids, most likely overnight by soaking them in hot water and laundry detergent. If you do this make sure to thoroughly rinse the grids before putting them back in to make sure you are not introducing soap/foam into the pool.
DE filters come in different sizes measured in sqft. For example my DE filter is 60 sqft and I put in the correct amount of filter media (DE or DE alternative) as described on the package for 60 sqft.
When backwashing you want to make sure you backwash until the water is clear and then rinse as well. At that point you will want to add only 80% of the described amount of DE.
