Forget about using a different commandline tool
in order to perform the update. It does not make a difference. The source of trouble has been the linuxmint repository server all the time and it still is.
Trying to connect packages.linuxmint.com timed out on 18th December for me. It did not time out but failed to get the inRelease file thus causing a GPG error
on 19th December mot of the day.
I have not changed anything on my system, in particularly nothing affecting network connectivity or updating software. So the source of the problem is not on my machine. Hence I can do nothing to solve or work around the Linux Mint repository problems whatever they are.Conclusion:
Those of us for whom updating works fine again are lucky. The rest of us can only hope that someone will get to the bottom of the problem and solve it.
Until then using Synaptic, mintUpdater, apt-get or aptitude does not make the slightest difference.
Currently I am downloading a 13.5 MB software package. Downloading is slow as good old snail mail in the 19th century. 1.6 kB/sec. I am not sure I will wait for more than an hour.
I'll try to be patient ...