XPS 13 9365 Thunderbolt Port Issue/WD15 Issue

Hi, community

This is a long story and I'm now totally fed up with this device, but still hoping if I could get some support from the community. I am using XPS 13 2-in-1 9365 and I also purchased a WD15 dock along with it. When I connected the dock to the laptop via the thunderbolt port (I know it's USB-C dock), I could see the dock charging the laptop battery, but other functions/ports(USB, Ethernet, HDMI...) couldn't be detected by the system at all. When I connected it to the USB-C port, everything worked fine, including charging. But the thunderbolt port still worked and could detect my thumb drive. So I thought it was a BIOS issue.

The original BIOS version was 2.9.0. I tried to download the driver from Dell's website and install it manually, the installation progress was smooth, except nothing happened after the system rebooted - it just continued to log in to Windows 10 directly. Also tried to update the driver via SupportAssist and Dell Command/Update and I got the same issue. So what I did was I made a bootable USB Win PE thumb drive, copy the latest 2.10.0 BIOS to the EFI folder and update the BIOS using BIOS downgrade/recovery function. It worked, and the BIOS version is now 2.10.0.

But here comes the new problems. When I connect WD15 to the thunderbolt port,  I can see the little LED light on the cable blinking (which it shouldn't) and the battery icon in the taskbar suggests the charging process was on and off and it keeps changing. Same thing happens after I connect it to the USB C port. Then I thought it might be a thunderbolt driver issue. So I download the latest thunderbolt controller and firmware from Dell's website. The controller, despite being outdated, can be installed, but the thunderbolt firmware update utility says I need to install/update the thunderbolt controller before it could begin.

Now the thunderbolt port cannot detect any device connected to it at all, but I could charge my laptop via that port. The USB C port works just fine, except when it's connected to WD15 dock.

Tried reinstall clean Win10 again and again and still got the same issue.

So...any thoughts?