"Best way of solving this issue... without hardware".
OK... but "the issue" is not particularly well-defined IMHO. For example:
clocks on the raspberry pi's were out of sync, causing the https request/response to fail
Assuming your RPis use the default
systemd-timesyncd, it's clear enough that system time on these RPis will tend to greater variance than some. This because
SNTP sacrificed accuracy for other objectives; see this for more details.
But... where is the https request/response failing? - on your client's machinery, or on your RPis?... or on your
django droplet thing-y? Available solutions will be different depending upon which hosts are choking over the bogus time. And is the
django droplet thing-y hosted on RPi - or on one of DO's servers? My point is only this: the missing details may mean you're foregoing some good solutions.
For example, one obvious solution is to replace
SNTP with a more accurate service in
chrony, and configure one of your RPIs to act as a time server for the others. But your question fails to inform us whether or not that's possible; i.e. "facility has restricted the pi's traffic to only the IP of my server". This is an ambiguous specification because you don't say if your "server" could provide (or relay) NTP services.
Another example: If your RPis can access an
https web server with accurate time, you could simply issue a
curl -v to that server, and
grep the headers for an accurate date-time:
curl -v https://google.com/ 2>&1 | grep "< date:"
< date: Sat, 31 Oct 2020 19:48:16 GMT
You'll obviously need to parse this quickly, and use it to set time on your system(s).
Similarly, perhaps a
django droplet can learn what the time is, and share that with your RPis.
Finally, wrt another fuzzy requirement:
Preferably without additional hardware.
What exactly is your priority? A working solution, or satisfying your preferences?
There are many sources of accurate time. You don't want hardware? OK - here's one solution:
call this number on your telephone: +1 (303) 499-7111
when your call is answered, enter the following command as fast as you can:
sudo date -s -u '2020-10-31 19:48:16'
OTOH, if hardware is allowed as part of a solution, you could consider a GPS-based solution - they make "GPS-HATs" for the RPi. If satellite visibility is an issue, consider using and RTL-SDR dongle to access one of the rf-broadcast time services.
An RTL-SDR dongle can receive accurate time broadcasts including ADS-B messages from aircraft and/or FDA facilities.
In the US, the WWV (and WWVH) signals are broadcast continuously on 2.5 MHz & 20 MHz frequencies. Around the world, there are other systems that provide time services. Besides the RTL-SDR dongle receiver, there is at least one excellent tutorial describing how to hack a receiver using a Raspberry Pi.
And so, if you exclude "additional hardware" from the set of solutions, you may be missing out on a world of inexpensive and reliable sources of time.