HI the reason for this MTU issue is very simple.
MTU means the largest size of data transmission via a network path
The MTU size id dynamic means that it can be several variable below a ciling size
This variable is auto detected by the ICMP protocol
The entire ICMP protocol family is blocked by TM Unifi.
IF TM unifi can identify which or their devices blocked the ICMP protocol than this issue wont arise anylonger
The ICMP protocol is responsible to auto detect the MTU than make use of the dynimaic setting to make the VPN work right away
About the VPN ...VPN protocl tends to load more header than the common way of cimmunication thus this is waht causing what you are experinceing right now
so:
1. Either TM allow ICMP protocol or
2. All TM Customers entire malaysia need to manualy adjust this MTU