the 'back' button issue isn't the same issue (i think) as the disconnection.
root cause is likely the secure server you are on when shopping.
possible fix is firewall reconfiguration. you may need to pinhole (port forward)
specific ports in the router, (possibly) disable integrated OS firewall, or create
advanced rules in 3rd party firewall. I have seen similar issues that were solved
by careful analysis of 'which ports' used during the transaction and creating rules to
allow.
Now, the disconnection is Possibly related but highly uncertain. For that, I'd start
by getting any info possible from the ISP tech support (2nd tier, Engineer level).
likewise, the router manual, and if available, firmware upgrade. Its very difficult to
analyze the two issues remotely. were I sitting in front of the computer, I would
start by examining firewall and router logs.. (save them, for comparison later) of
the times in which problems occurred. Pre-gathering all info from ISP, router
FAQ/Troubleshooting pages, the manual, and possible firmware upgrades, also.
It's fixable. you may need a different router, unlikely. a different firmware, quite
possibly. a different firewall and/or router configuration, VERY possibly.
more specific info, (OS, Router Model, NIC model, age of components, ISP, signal
quality) isn't wrong either, for diagnosis. Ya got a nice complex one, but likely
the fix is simple once you figure out what it IS.