Take it out the pot and firstly examine for vine weavils, chubby fat white grubs with a brown head. If you have those than you will need to use vine weavil killer in all your pots and boxes
Get new rose compost and micorrhizal fungi,. Trim roots to half. Cut top down to half. Re-pot only up to and not covering the scion, join between rootstock and rose (the nobbly part at the base. Put pot in a shady place on risers and water so that soil is only moist
If in doubt about the rose being alive, use fingernail and scrape a bit of bark away low down. If there is any green then rose is still alive
I agree with the posts recommending planting in the ground but do the above first.