Даже 2 из 4 их ns не резолвятся через впн, хотя без впн всё норм:

$ nslookup ns1.selectel.ru 192.168.104.1
Server:		192.168.104.1
Address:	192.168.104.1#53

** server can't find ns1.selectel.ru: SERVFAIL

$ nslookup ns2.selectel.ru 192.168.104.1
Server:		192.168.104.1
Address:	192.168.104.1#53

** server can't find ns2.selectel.ru: SERVFAIL

$ nslookup ns3.selectel.ru 192.168.104.1
Server:		192.168.104.1
Address:	192.168.104.1#53

Non-authoritative answer:
Name:	ns3.selectel.ru
Address: 188.68.203.5
** server can't find ns3.selectel.ru: SERVFAIL

$ nslookup ns4.selectel.ru 192.168.104.1
Server:		192.168.104.1
Address:	192.168.104.1#53

Non-authoritative answer:
Name:	ns4.selectel.ru
Address: 77.223.114.10
** server can't find ns4.selectel.ru: SERVFAIL

$ dig @192.168.104.1 ns1.selectel.ru

; <<>> DiG 9.16.44-Debian <<>> @192.168.104.1 ns1.selectel.ru
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 25214
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;ns1.selectel.ru.		IN	A

;; Query time: 2200 msec
;; SERVER: 192.168.104.1#53(192.168.104.1)
;; WHEN: Sun Nov 05 15:58:51 MSK 2023
;; MSG SIZE  rcvd: 44

$ dig @192.168.104.1 ns2.selectel.ru

; <<>> DiG 9.16.44-Debian <<>> @192.168.104.1 ns2.selectel.ru
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 34306
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;ns2.selectel.ru.		IN	A

;; Query time: 68 msec
;; SERVER: 192.168.104.1#53(192.168.104.1)
;; WHEN: Sun Nov 05 15:58:54 MSK 2023
;; MSG SIZE  rcvd: 44

$ dig @192.168.104.1 ns3.selectel.ru

; <<>> DiG 9.16.44-Debian <<>> @192.168.104.1 ns3.selectel.ru
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 22177
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;ns3.selectel.ru.		IN	A

;; ANSWER SECTION:
ns3.selectel.ru.	16759	IN	A	188.68.203.5

;; Query time: 68 msec
;; SERVER: 192.168.104.1#53(192.168.104.1)
;; WHEN: Sun Nov 05 15:58:57 MSK 2023
;; MSG SIZE  rcvd: 60

$ dig @192.168.104.1 ns4.selectel.ru

; <<>> DiG 9.16.44-Debian <<>> @192.168.104.1 ns4.selectel.ru
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 30223
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;ns4.selectel.ru.		IN	A

;; ANSWER SECTION:
ns4.selectel.ru.	9860	IN	A	77.223.114.10

;; Query time: 68 msec
;; SERVER: 192.168.104.1#53(192.168.104.1)
;; WHEN: Sun Nov 05 15:59:02 MSK 2023
;; MSG SIZE  rcvd: 60

На текущий момент проблему не наблюдаю ни на одном VPN-сервере.

Да, сейчас всё работает. Но где-то была проблема. Какие upstream dns у антизапрета?

Никаких, это рекурсивный резолвер.