Thanks for your replies - I have decided not to run Debian on this laptop any more for this and other reasons.
I have resolved this issue by replacing 32 bit and 64 bit Debian 10 LxQt with MX 18, 32 bit and AntiX 19, 64 bit.
Deb-fan wrote:system model/specs have zero to do with this.

I Agree on this point thanks - all those icons work in other desktops and versions of Debian on this and other computers. I had another issue with the computer freezing (not running slowly) white screen it seems to be due to over heating when the processor was working hard such browsing a slow webpage or pdf so I have been evaluating MX 18, 32 bit and AntiX 19, 64 bit which don't have any of these issues.
So the point about the old laptop being the issue - I don't think there is a problem hot air flows through well and with AntiX for example the processor clock can run at 2.2GHz (maximum) with hot air flow out but not so hot that the laptop closes down and restarts with a brief message overheating that occurs with Debian. That is all tray icons including both the Connman wireless icon and the Mega.nz icon are displayed in AntiX and MX distributions.