A quick look on eBay shows a lot of offerings for vacuum tube shields but I do not have any personal experience.
https://www.ebay.com/sch/i.html?_fr....m570.l1313&_nkw=vacuum+tube+shields&_sacat=0
What I did myself with other amps was to take aluminum foil and shape it around the tubes. Not pretty, but sometimes helped. There is something called a Faraday cage - basically a metal cage. If you have a metal pen holder cup or similar you could put it over the tubes to see if it helps.
I know of somebody who took a paperback and leaned it against the tube and it helped!
Or plugging in the offending router a distance away if possible....