Here is the EE Page that describes the general process-
I worked through that about 4 weeks back, successfully setting up a secondary SSID.
For various reasons, I did not maintain it, including there were some problems accessing some of the local WiFi items I wanted to use - whether this was caused by me or is a fundamental aspect of the design, possibly someone with more knowledge may be able to help further.
Thinking more of your situation, as I posted before, protect the Brightbox immediately by changing the ADMIN Password and possibly the ID Name.
To avoid the kids getting any inkling, I suggest that you leave SSID1 "as-is" apart from putting limits on the Bandwidths.
That way, there is less likelihood of them becoming aware of any changes.
I suggest that its Download Band Width should be not much more than presently achieved in practice.
Set up SSID2 for your own use, giving it a Download Bandwidth nearer to what you think/are advised is achievable on your line, so that effectively your use is prioritised automatically whenever you are using it; and it is "released back" to the kids when you are not using it.
I suspect that my access problems may have been my choice of "VLAN Binding", where I selected "VLAN2" as opposed to "Default"
On inSSIDer, I noted that generally the graph of the two SSIDs were superimposed, although I think on one or two occasions, SSID2 may have been reduced to the Lower/Primary Channel, 7 from 7+11
Again, hopefully someone with greater knowledge may be able to enlarge on those aspects.
I note that you have not posted the statistics some have requested.
This forum can be extremely helpfull on such matters.
The main statistics come under the ADVANCED group, just as the above additional SSIDs do.
Login as ADMIN (if you have not changed it)
COPY that first page top section, "Internet Settings"; and paste in to this forum, possibly via an intermediate WORD/similar document.
Select at the top, "Advanced Setup"
In the left-hand column, select "System", last item on the list.
On the "SYSTEM" page, select the second-last item iin the left-hand column "ADSL Status"
Again COPY and PASTE ALL of the information on that page/screen, from-
The main information that is being requested are the four Attenuation figures in the second grouping headed "Defect/Failure Indication", as these reveal a lot about the basic condition of your phone line, including approximate wire/cable distance from your Exchange.
You can be literally "next door" to your Exchange; but the wire may go from the exchange to a distribution cabinet some distance away, 500 metres, 2000 metres etc
And then come back to the local cabinet, and onwards to your house.
Even that last part may be strange.
The local exchange-to-cabinet cable passes by the side of my house, through an obvious, three-panel, underground jointing chamber, and across the road to the cabinet, about 25 yards.
But the connection from the cabinet to my house first goes eastwards to the far end of a near-neighbours house, to a small, inconspicuous Jointing Box at footpath level, before coming back westwards to my house.
That is, a foray of at least 125 yards, to cover a geographic, straight-line distance of 25 yards.
The other data help build up a broader picture.