I'm sorry to hear that. It must frustrating and stressful for you. In case this is helpful at all, I believe the government tried to support voting arrangements for residents of care homes the last time there was some kind of election, but I'm afraid I don't remember if that was before Covid. If you contact your local friendly DAB representative and ask them if there's any special arrangement or help for care home residents who wish to vote but can't leave, they may be able to figure something out.
Generally, in the past, the government has been quite keen to help care home residents and elderly voters get to cast their votes.
You've probably looked into all this already and there may not be any workarounds!