It sounds like you are very close to making this work. You simply need to create VLAN 2 on the switch and assign ports to each VLAN.
Go to VLAN Managgement-> Create VLAN and Add.
VLAN ID: 2
VLAN Name: Guest
Go to Port VLAN Membership and click Join VLAN... at the bottom. Select port 26 from the dropdown. In the right box, select 1UP and use the left arrow to move it to the left box. Then select 2 from the left box and choose Untagged at the bottom. Use the right arrow to move 2UP to the right box. Now port 26 is a member of VLAN 2 untagged.
Do the same for any other port that you want to be a member of VLAN 2.
You can also create a Trunk port between the WRVS4400N and SG200 which will pass both VLAN 1 and 2 over the same connection. In that case on both the router and switch make the port a trunk and add VLAN 1 Tagged and VLAN 2 Untagged.
I was able to create the "Guest" without a problem.
However the rest is where I get confused. I have be wrong but I expected to run cat6 from port 1 on the WRVS4400 to port 25 on the SG200 and another cable from port 2 (public) on the WRVS4400 to port 26 on the SG-200. Then configure the ports 13 - 18 a VLAN 2 Guest or Public.
Also when I go to "Port VLAN Mambership" I do not have the option to "Join" VLAN. I do have the option under VLAN to Port.
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