What is the best product for reapplying the DWR?
Grangers has worked pretty well for me. So has Nikwax. But I think the Granger's is slightly better.
Unfortunately none of the DWR's last as long as the manufacturer applied products.
If the jacket has thermoset trimmings, around pockets, etc. Be careful of the dryer heat (it can soften the glue) otherwise dry it as hot as possible, limited by the manufacturer recommendations.
If you have hard water. The minerals in the water can be deposited in the pores of the membrane and decrease performance, same as some detergents.