Can I perform Kartik Poornima rituals at home if I cannot go to a river?

Yes. Add a small quantity of Gangajal to your bathwater and take your pre-dawn bath at home with the recitation of the Ganga Stotra. Set up a clean puja space, light a ghee lamp, and perform the Vishnu Puja with Damodara Ashtakam. Place a lamp near the Tulsi plant. You may float a lamp in a bucket of water or a local pond as a symbolic Ganga offering. Simultaneously, have Prayag Pandits perform the Ganga Snan and Deep Daan on your behalf in Varanasi.