PPPoE MTU issue

Hello,

Tech info:
Internet: 100Mb/s, optic, both sides
Router: RB3011UiAS-RM both sides
Subnet: 192.168.0.0/24 both sides
Ping between local and remote offices: ~20ms

We have EoIP enabled between our local and remote offices.
On both sides we have PPPoE connections.
We expecting issues with opening of some websites, and ask for help to how to hange the MTU on both sides to 1400 or help to fix this issue.
In “IP->Firewall->Mangle” section present two pre-default rules(non-changable “gray”):
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With default non-changable values:
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Also we have slow motion when people working using RDP, it looks like a bit slow redraws the screen when using web browser, ms office, etc.
Please advice how to remove issue with opening of some websites and increase the speed between two offices.
Thanks in advance.


Regards,
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