Thank you @meyay for this guidance.
EDIT: accidentally pressed Ctrl-Enter and didn’t type the complete response. See below
Indeed there is a compose file with an extra container for nordvpn (where I specify the network you correctly said is not shown here) and indeed there is a variable for specifying allowed IPs to access that VPN contaienr. The problem was that I had mistyped the 192.168.43.0/24 with a comma instead of a dot between 43 and 0, which apparently caused all this pain.
I sincerely appreciate your response, I had given up on finding this. I had actually checked this already but the dot/comma for my age looks identical. After your suggestion I paid close attention and spotted my typo. Marking this as solved by your reply.