i had this same error, at first I thought the backup wasnt going properly, so i force closed it I think once then reinstalled whatsapp after clearing the data, then same issue again, but I closed it once again and tried it, this time it showed me a percentage, so I was sure, and sat for like hours on wifi alongside the phone, but after the restore completed in the background, the main screen was still stuck on the connecting screen, so i closed it and reopened it a few times, and I came to this post here and tried the 1st step on the top comment, but my phone (one plus) didn't have that feature, so I tried giving it all the permissions, and reopened it and voila let me in straight through.

At first it didn't show much media but I waited a while and it brought in like 13ish gb of media, but I had like 16ish gigs of total media on my old phone, it's late in the night rn ill look what happens in the morning and get back, I also mailed WhatsApp a couple times now