Thank you @AntoineBD,
Sorry I took so long, but I decided to upgrade my Zyxel switch to the latest firmware before trying out your suggested solution with VLAN 1977.
However, it took me some time because the default IP address was 192.168.1.1 and it proved difficult to reach the WEB configurator. Happily, I found a post on REDDIT which helped me set the IP address to 192.168.1.2. Then I successfully upgraded the firmware. And it seems to work fine.
However, I can find nothing in the configuration option in the WEB configurator that look anything like your suggested changes. Pity.
Maybe I should buy the Swisscom switch, but it does seem to be a rather old tp-link device.
thanks robert