Can I access my photos on iCloud if my device is lost

it depends what securities were set in iPhone , if activation lock was set up https://support.apple.com/en-gb/HT201365 ( so that old iPhone could not be opened ) , use a web browser and sign in with apple id and password at www.icloud.com to view photos https://support.apple.com/en-in/guide/icloud/mm73262d11fe/icloud

Using iCloud will only work if you had set the applicable files to save to iCloud when you still had the lost phone. If you open the Photos app on a Mac you can usually download whatever is stored in iCloud from any device that you previously had set to save to iCloud.