l2tp MTU+MRU 1418, MSS 1378, в firewall в самом вверху добавил разрешающие правила на любое соединение в сторону l2tp (цепочка forward).
[admin@MikroTik] > /export compact
# sep/06/2018 09:48:38 by RouterOS 6.42.7
# software id = B3AV-ZEIV
#
# model = RouterBOARD 750G r3
/interface bridge
add admin-mac=CC:2D:E0:06:67:E8 auto-mac=no comment=defconf name=bridge
/interface l2tp-client
add allow=mschap2 connect-to=81.81.81.81 disabled=no ipsec-secret="abcdefghijk" keepalive-timeout=disabled name=l2tp-out1 password=psw use-ipsec=yes user=usr
/interface list
add comment=defconf name=WAN
add comment=defconf name=LAN
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
/ip hotspot profile
set [ find default=yes ] html-directory=flash/hotspot
/ip ipsec proposal
set [ find default=yes ] enc-algorithms=aes-128-cbc,3des
/ip pool
add name=default-dhcp ranges=100.1.15.10-100.1.15.250
/ip dhcp-server
add address-pool=default-dhcp disabled=no interface=bridge name=defconf
/port
set 0 name=usb1
/interface ppp-client
add apn=internet disabled=no info-channel=1 name=ppp-out1 port=usb1
/interface bridge port
add bridge=bridge comment=defconf interface=ether2
add bridge=bridge comment=defconf interface=ether3
add bridge=bridge comment=defconf interface=ether4
add bridge=bridge comment=defconf interface=ether5
/ip neighbor discovery-settings
set discover-interface-list=LAN
/interface list member
add comment=defconf interface=bridge list=LAN
add comment=defconf interface=ppp-out1 list=WAN
/ip address
add address=100.1.15.254/24 comment=defconf interface=bridge network=100.1.15.0
/ip dhcp-client
add comment=defconf dhcp-options=hostname,clientid disabled=no interface=ether1
/ip dhcp-server network
add address=100.1.15.0/24 comment=defconf gateway=100.1.15.254
/ip dns
set allow-remote-requests=yes
/ip dns static
add address=192.168.88.1 name=router.lan
/ip firewall filter
add action=accept chain=input comment="defconf: accept established,related,untracked" connection-state=established,related,untracked
add action=drop chain=input comment="defconf: drop invalid" connection-state=invalid
add action=accept chain=input comment="defconf: accept ICMP" protocol=icmp
add action=drop chain=input comment="defconf: drop all not coming from LAN" in-interface-list=!LAN
add action=accept chain=forward comment="defconf: accept in ipsec policy" ipsec-policy=in,ipsec
add action=accept chain=forward comment="defconf: accept out ipsec policy" ipsec-policy=out,ipsec
add action=fasttrack-connection chain=forward comment="defconf: fasttrack" connection-state=established,related
add action=accept chain=forward comment="defconf: accept established,related, untracked" connection-state=established,related,untracked
add action=drop chain=forward comment="defconf: drop invalid" connection-state=invalid
add action=drop chain=forward comment="defconf: drop all from WAN not DSTNATed" connection-nat-state=!dstnat connection-state=new in-interface-list=WAN
/ip firewall nat
add action=masquerade chain=srcnat comment="defconf: masquerade" ipsec-policy=out,none out-interface-list=WAN
/ip route
add distance=1 dst-address=100.1.0.0/16 gateway=l2tp-out1
/system clock
set time-zone-autodetect=no time-zone-name=Asia/Yekaterinburg
/system routerboard settings
set silent-boot=no
/tool mac-server
set allowed-interface-list=LAN
/tool mac-server mac-winbox
set allowed-interface-list=LAN
[admin@MikroTik] >