I wouldn't update the firmware to v28 as it blocks access to the Supervisor password.
When your up and running we can help you get the Supervisor password, then you can upgrade firmware.
Keep this simple. Start again.
Reset the unit. Poke something suitable like a paper clip in the reset hole while the unit is powered on. You will hear/feel a button being clicked. Keep this pressed until the lights on the unit change.
You only need to change a single section to get online.
Kitz has a good guide with an image of suggested settings. IPv6 isn't used on Plusnet but it shouldn't hurt to enable it as in kitz guide.
https://kitz.co.uk/routers/zyxel_VMG8324-B10A_vdsl_setup.htmNetwork Settings, Broadband.
WAN Configuration Edit
Active Tick
Name : VDSL
Type : VDSL OVER PTM
Mode : Routing
Encapsulation: PPPoE
IPv6/IPv4 Mode: IPv4 Only
PPP Information
PPP User Name : username@pluses.net (put your username)
PPP Password : your Plusnet password
PPP Trigger Type : Auto Connect
Authentication Method: Auto
IP Address
Obtain an IP Address Automatically
NAT Enable : Tick
Apply as default gateway: Tick
VLAN
Active : Tick
802.1p : 2
802.1q : 101
MTU Size : 1492
If you wish change Wireless passwords.
Telnet can be enabled under "Maintenance > Remote Management".
The rest of the router is pretty advanced and doesn't need touched unless you know what you're doing.
I think you may have been confusing IGMP and ICMP above. ICMP is for responding to pings.
IGMP is for YouView multicast IPTV channels.
I would leave the IGMP settings well alone.
edit: too slow at typing