Which firmware version is your hub running? I had issues in modem mode with my hub 4 - mine was running :
There’s a newer version which my hub hadn’t received which supposedly addresses some of the modem mode issues:
AR01.04.055.06.07_040722_7245.PC20.10.LG.X1
With the newer version the light ring turns green in modem mode.
Virgin sent an engineer as my hub wouldn’t update. She replaced my hub 4 with a hub 5 last week - so far this has worked fine in modem mode.