Dried semen

Assalamu Alaikum, I remembered that I was impure 4 months ago and I think I had semen on my hands and I sat and touched the office chair and keyboard. Are they now considered still impure even after they are dry?

Back then, I was sure it was pure but now I am not. Just to be on the safe side, how shall I wash the keyboard and office chair and have I become impure again as I’ve sat on the office chair which might be impure and do I need to do ghusl again?

Jazak Allah.