It's up for me. You may be in an area, or using an ISP, that is blocking access to that domain. Unfortunately I don't have any info to hand on alternate access methods, but I know they exist.

Works for me, check if sci-hub.se resolves to e.g. 186.2.163.219.

Or just:

curl --resolve sci-hub.se:443:186.2.163.219 --remote-name https://sci-hub.se/downloads/2020-12-18/44/10.1126@science.3...

If it doesn't work you can try try replacing .se with .st or .do