Problem #1: With the new iOS 14 update now the VPN I configure is not working it keeps showing:
"The l2TP-VPN server is did not respond. try reconnecting. if the problem continues, verify your settings and contact your admin."
but if I try on my other iphone which runs IOS 13.7 it connects fine with the same exact settings. (sourced: https://developer.apple.com/forums/thread/660499)

Cause #1: "We have upgraded the proposed ciphers in L2TP IPsec VPN to also propose SHA-256 for the Child SA in IPsec. The issue seems to be that the server is accepting SHA-256 cipher for the child but maybe dropping the ESP encrypted packets with SHA-256 HMAC. This maybe because the server is assuming a SHA-256 HMAC with 96 bits instead of the standard 128 bits. Switching the SHA-256 HMAC output from 96 to 128 bits should fix this issue."  by Matt Eaton, DTS Engineering, CoreOS
meaton3@apple.com.

Resolution #1:  Update IOS to 14.1 or latest or downgrade to 13.7 which will resolve the problem. Alternatively, go to Apple Shop in order to resolve IOS compatibility issue.

Problem #2: L2TP/IPSec is connected but still failed to access oversea sites.

Cause #2: This is caused due to iPhone IOS do not allow further any update of cellular DNS server information, or selected "Send All Traffic: OFF" during the L2TP/IPSec setting.

Resolution #2: 
1) Goto Settings > General > Network > VPN > Select "Send All Traffic: On" during the L2TP/IPSec setting.


Alternatively Resolution #3
1) Download > DNS Override (https://www.dnsoverride.com/) or (https://itunes.apple.com/us/app/dns-override/id1060830093?mt=8)

Installation procedure is as follows > https://www.dnsoverride.com/change-iphone-dns-with-an-app/

2) Download > 1.1.1.1 (https://apps.apple.com/hk/app/1-1-1-1-faster-internet/id1423538627)


install it to resolve DNS problem which unlock the DNS limitation from your local ISP network provider.

Good luck!