I thought I'd post this, if it can help someone. I wasn't able to download the latest update with March Security Patch on my S21 Ultra.
It'd just get stuck on "downloading" and wouldn't actually download anything.
Samsung rep over the phone tried everything remotely, but couldn't really fix it. According to him, my phone wasn't able to connect with Samsung servers to download the update.
I faced the exact same issue on my S20+, and I had to go to Samsung service center to get it fixed.
SOLUTION : I just tried downloading the firmware (OTA Update) using a different WiFi, and it actually worked. I was able to download and update without any issues.
