Hi Lee, thank you for your concern. I thought marking Carl's response as a Helpful Answer would close the matter.
Since you prod me 🙂 I must reiterate:
I haven't seen mention anywhere of the Base LED being purple/magenta in Modem Mode. As I for one can't see a difference from red, it seems a crazy choice to have made- I waited a long time thinking there was a temporary "error condition" while the hub was changing mode.
Secondly, is overheating the only fault condition that the Hub3 can indicate when in MM?
[As it happens I have an engineer visit booked for tomorrow for what telephone support called an "intermittent connection" (my new modem has not yet exceeded 7h40 uptime). However, I believe this fault to lie outside the premises.]
The tech visit should be able to shed some light and I'm confident will resolve your issues. I've not looked at your hub remotely so I cannot tell if there's anything to suggest there's an external issue.