same... it seems the unifi always like that!

Sometimes it is the router that does not work well!

Temporarily solution, I did, (if just facing wifi issue, not LAN), if have unused old streamyx wifi router (A),
I just set it to bridged mode, plug into the L7 router through LAN.
So wireless device connect to A wifi, then through LAN to L7.

BTU is a modem, not a router.
So 2 router, but one acts as switch.

BTU is a modem, not a router.
So 2 router, but one acts as switch.

Yes, B acts as access point.
Just like what I had mentioned.
B connect to A via LAN, while B has wireless capability.

Yes, B acts as access point.
Just like what I had mentioned.
B connect to A via LAN, while B has wireless capability.

IPTV has dedicted port from the A (a VLAN tagged router that given), aka normally at port number 4.

The given router has 4 port, just plug the B router via LAN(bridged router that act as access point), into either 1-3 port, but not 4.
While the STB is connecting to port 4.

BTU ---> VLAN tagged router ->

--> Bridged router (access point). (port 1)
--> STB (port 4)

I cant access to huawei hg8240w, anyone can help?

Edit: I can't access the GUI

You can't connect to BTU (fiber Huawei HG), when you computer is connected to L7 router.

You need to plug in directly from your computer LAN to the modem, aka bypass the L7.

Why you want to go into the BTU?
If want to reduce to 2, then get a VLAN router to replace L7 and Linksys.
What you can do on the BTU to reduce it to 2? 

I don't quite understand your situation/description.

At current setup or ordinary setup
BTU --> L7 --> your computer. Surely you cannot access into the BTU.

You want to access into BTU
BTU --> your computer. But the BTU has no DHCP and routing function.

I do not know why you have linksys wireless or how you connect the linksys wireless.

The BTU is not a router.

It's possible to reconfigure the VDSL BTUs as well as some of the GPON ones to handle VLAN tagging/untagging for you (basically allocate one port as untagged from VLAN 500) so you only need a PPPoE capable router.. however if I'm not mistaken, the GPON BTUs now lock users out of the web UI so you won't be able to reconfigure VLAN tagging/untagging there.

The Huawei BTUs are capable of handling PPPoE, NAT as well as DHCP -- it's already doing this for voice traffic. It was possible to do this easily back in 2011 but it's not really practical anymore IMO