Try
set vpn l2tp remote-access mtu 1360 set vpn l2tp remote-access outside-address 0.0.0.0 set firewall options mss-clamp interface-type all set firewall options mss-clamp mss 1400
Try
set vpn l2tp remote-access mtu 1360 set vpn l2tp remote-access outside-address 0.0.0.0 set firewall options mss-clamp interface-type all set firewall options mss-clamp mss 1400