Xubuntu LTS 18.04 fully updated, on Dell laptop. New problem within the past week, probably following an update.
After resuming from suspend, network manager shows I am still connect to my wireless router, but I have no connection to the internet. From another device I can confirm that the router is working normally. A reboot restores normal operation of the laptop.
This thread, https://ubuntuforums.org/showthread.php?t=2384252, now closed, contains some solutions which I have tried. However, I have noticed that running (as root) the command
echo 1 > /sys/bus/pci/rescan
after waiting until several minutes from resuming does usually restore the connection, but does nothing if I run it during the first few minutes after resume. It's a great nuisance, and since I had no problems for the 18 months I have run the system, I believe the bug has been introduced by a recent update. I installed a new update yesterday, but that did not fix it.