dwalton
I can access both those sites using Firefox OK, from behind a sme 8.1 server using ADSL2+ in Australia.
I can access them OK from another location via mobile.
Your tests are suggestive that sme server is not involved in this problem
ie another completely different OS has the same issue, so it is likely an external issue or a configuration issue.
You also said you made no changes to your sme server when the problem occurred, so something external happened.
Have you googled 408 errors, the answers are suggestive the problem is timimg issues with the website, but timing issues could happen anywhere in the route.
Also you did not answer one of my earlier questions
ie
"I would also ask, do you have a specific DNS server setup in your sme server, look in server manager panel to see, if you are unsure ?"
Look in server manager, configuration, Domains, Modify Corporate DNS settings.
There should be (in most situations) no DNS servers shown there, as sme server is capable of resolving requests itself.
If there are DNS server(s) shown, then maybe those are having problems with resolving staples & depot sites in question.
In either case, as you say you have success when connecting directly to your ISP via the router, then you could try setting the same ISP's DNS servers here, & see what happens then.
Check your router or ISP site for details of their DNS servers.
As I said earlier, & which has been reported many times here over the years, DNS servers can have problems even with just one specific website, for a variety of reasons ie stale info etc.
What is your location & who is your ISP, perhaps others who use the same ISP or are in your location can test those problematic sites.
You could also try setting up a sme8.1 server & leave all settings at defaults except the basic settings you need to make to attach to your network.