It is possible that the ants are indirectly causing the decline of your shrubs by causing damage to the roots and creating large air pockets around them. The small feeder roots may be damaged or may not be in contact with the soil for nutrient and water absorption.
You are correct that you need to try to kill the ants. I don't know what you have tried but I would suggest trying Bonide Ant, Flea & Tick Killer Granules or Bayer Advanced Complete Ant Killer Plus according to the label directions.
Once the ants are gone, water the shrubs well and feed them with Espoma Holly-tone according to the label directions. It might be good to apply some Espoma Triple Super Phosphate according to the label to help with root growth.
They should begin to recover but it may take some time once the ants are gone.