I am running the same SWG as you are
As was said, the AquaTrol RJ does not have a flow switch, and therefor has to be installed in a vertical position where the return enters the pool. The reason being that when running, gas builds up in the cell. With the unit in the vertical position right below the return, even if the pump stops running, the gas can vent up and out of the return. If the cell is in any other sort of configuration, such as horizontal, or with some sort of loop above it (as in your install) you are supposed to have the flow sensor so that if the flow stops, the unit turns off.
Keep in mind that the RJ requires a minute or two upon start up to realize it is an RJ. When first turned on, the panel will report a no flow error. After a minute or two it realizes that it does not have a flow sensor and then starts working.
I have a valve between the cell and my filter. If I have to remove my cell, I plug the return (with the expandable rubber plug as above) and then close the valve on the other side.
For the record, in my pool (which is 17,000 gal) I run my SWG 8 hours a day, at 80% capacity and it keeps the FC where it needs to be. I do break it into two 4-hour runs. AM and PM. I also keep a solar cover on my pool when it is not being used, and my pool is not used by tons of people for hours at a time, so that is something to consider, buy the T5 does its job for me.