from; https://www.draytek.co.uk/archive/kb_vigor_dns.html

By default, if your ISP allocates its own DNS servers when the router logs into the ISP, these will then over-ride the manual settings.

If you want to force the manual settings to apply, from telnet you can use this command:

srv dhcp frcdnsmanl on (use 'off' to disable it. Requires f/w 2.5 or later).

like this;

DrayTek> srv dhcp frcdnsmanl on
% Domain name server now is using manual settings!

In more recent firmware (September 05 onwards, varies with model) a new command has been added which supercedes the above command:

DrayTek> ip dns 208.67.222.222 207.67.220.220

https://shop.seg.co.uk/globalview-b.html

https://community.tp-link.com/en/business/forum/topic/89721?replyId=215336

  If you like 2 or more VLANS you must make this, port 1-24 all PVID1, VLAN2 port 2-4 PVID2, VLAN3 port 5-8 PVID3. This allow all ports from 2-8 to connect with port1, router, but port 2-4 not allowed to conect with port 5-8 or 5-8 with port 2-4

So you tag ingress traffic on ports 2-4 with VLAN ID 2 and ingress traffic on ports 5-8 with VLAN ID 3. Egress traffic to port 1 will have the tags stripped, but port 1 has to be a member of VLAN1, 2 and 3 to have the switch forward packets arriving on ports 2-4 or 5-8 to port 1.

Ingress traffic arriving on port 1 is tagged with VLAN ID 1 and forwarded to all ports if they are in the default VLAN 1. On output, the tags are removed if ports are untagged.

Now on the SG108E a device on port 2 will reach any other device on all ports of the T1600, no matter which PVID is set by the SG108E if you use untagged output on port 1 connected to port 21 of the T1600. Try it.