Well, the Dell service guy fixed it  Installing the ASMedia USB extensible Host Controller Driver" solved all problems. All my TB3 devices are working fine now under BIOS version 1.20.2 Thanks again for your speedy help Cheers!
Hello there...After updating my 7740 to latest BIOS 1.20.2 the TB3 port is not working properly. I had a WD19T docking (with 240W power supply) connected, which did not charge the 7740 anymore. Also, no USB stick or connected LAN was working. First thing I thought is that docking is gone to nirvana, so I replaced it by a brand new one. Nothing changed... Also my external Samsung X5 connected directly to TB3 port was not recognized any longer.I finally downgraded the BIOS 1.20.2 to 1.18.1 and guess what...? Yep, everything is working fine again! Thanks to them Dell BIOS programmers (which are not really doing a good job... remember the UHD issues last year) I bought another WD19T for just nothing... Cheers!
Precision 7740 - Thunderbolt not working after BIOS 1.20.2 update