Help: Can't connect to ISP router

[m8mu25]

Synopsis: I can't connect my OPNsense box to my ISP's router.  I need my ISP's router for TV services.

Equipment:
1. VZN (ISP's router)
  - Hardware: Verizon CR1000A
  - Ports: 4; WAN, 10Gb LAN, LAN 1, LAN 2
  - Firmware: 3.2.0.14 (latest)
2. OPN (OPNsense box)
  - Hardware: Protectli Vault FW2B
  - Ports: 2, WAN and LAN
  - OS: OPNsense 23.7 commit b35678139 (no minor updates, yet)
3. Switch (Managed switch)
4. WAP (Wireless access point)
5. RPi (Raspberry Pi for management)
Note that WAP is mentioned for completeness but is irrelevant.

Topology: ONT/Modem --- VZN --- OPN (and RPi) --- Switch --- WAP (and RPi)

OPNsense interfaces:
- [WAN] assigned to OPN's WAN port
- [LAN] assigned to OPN's LAN port
  - [LAN] is the parent of all VLANs and is not used directly.
- [VLAN30] for management
- [VLAN40] for users to connect to the Internet over Wi-Fi (irrelevant)

Networks:
10. 10.10.10.0/24 (Verizon)
  - VZN: 10.10.10.1
  - OPN [WAN]: 10.10.10.2
  - RPi: 10.10.10.10
  - Set-top boxes
20. 10.10.20.0/24 (Not used directly)
  - OPN [LAN]: 10.10.20.1
30. 10.10.30.0/24 (Management)
  - OPN [VLAN30]: 10.10.30.1
  - Switch: 10.10.30.2
  - WAP: 10.10.30.3
  - RPi: 10.10.30.10
40. 10.10.40.0/24 (Wi-Fi)
  - OPN [VLAN40]: 10.10.40.1
  - Future user devices

Bad behaviors:
1. I CANNOT access Verizon's web GUI (https://10.10.10.1) from RPi (10.10.30.10) connected to Switch.
2. Verizon's web GUI does NOT show OPN (10.10.10.2) under Devices.

Good behaviors:
3. I can connect RPi (10.10.30.10) to Switch and access the OPNsense web GUI (https://10.10.30.1).
4. I can connect RPi directly to OPN LAN port and access the OPNsense web GUI (when System > Settings > Administration > Web GUI > Listening Interfaces includes [LAN]).
5. I can connect RPi (10.10.10.10) directly to a VZN LAN port and access Verizon's web GUI (https://10.10.10.1).
6. Verizon's web GUI shows RPi under Devices.
7. Verizon web GUI shows that the VZN LAN ports used for OPN and RPi are connected.
8. The LEDs on the back of VZN, OPN, and RPi all seem to indicate connections:
  - LAN ports (to both OPN and RPi) on VZN are solid white on the right for a few minutes and then turn off.
  - WAN (to VZN) and LAN (to Switch) ports on OPN are solid green on the left and blinking orange on the right.
  - RPi port (to VZN or Switch) is solid green on left and blinking orange on right.

OPNsense settings of note?:
- All 4 interfaces (LAN, WAN, VLAN30, and VLAN40) are enabled.
- `Block private networks` and `Block bogon networks` are unchecked for all interfaces.
- `IPv4 Configuration Type` is set to `Static IPv4` for all interfaces.
- IP addresses and masks:
  - [WAN]: 10.10.10.2/24
  - [LAN]: 10.10.20.1/24
  - [VLAN30]: 10.10.30.1/24
  - [VLAN40]: 10.10.40.1/24
- Services > DHCPv4 is only enabled for [VLAN40].
- DNS servers are only given in Services > UnboundDNS > DNS over TLS.
- System > Settings > General > Networking > Allow DNS server list to be overridden by DHCP/PPP on WAN is unchecked.
(My goal is to redirect all traffic on port 53 to the local UnboundDNS service and resolve DNS requests with the given external servers.)

OPNsense firewall rules:
- No NAT rules are manually set.
- No firewall rules are manually set under Floating, [Loopback], [LAN], nor [WAN].
- [VLAN40] has rules analogous to [VLAN30] minus the rules to allow RPi to access the web GUIs (rules 3, 4 and 5).
- [VLAN30] rules, where PrivateNetworks is an alias for private networks (e.g. 10.0.0.0/8), loopback (i.e. 127.0.0.0/8), bogons, etc.:
1. UDP, VLAN30 net -> VLAN30 address:DNS
2. UDP, VLAN30 net:NTP -> VLAN30 address:NTP
3. TCP, 10.10.30.10 -> VLAN30 address:HTTPS
4. TCP, 10.10.30.10 -> VLAN30 address:HTTP
5. TCP, 10.10.30.10 -> 10.10.10.1:HTTPS
6. Block, IPv4+IPv6, VLAN30 net -> PrivateNetworks
7. TCP, VLAN30 net -> !PrivateNetworks:HTTPS
8. TCP, VLAN30 net -> !PrivateNetworks:HTTP

References:
1. https://homenetworkguy.com/how-to/set-up-a-fully-functioning-home-network-using-opnsense/
2. https://homenetworkguy.com/how-to/use-opnsense-router-behind-another-router/