Thanks for your help.
Since I do not use routing marks I removed the "gateway" part, which detects the internet interface and I added static resolution of the interface like the 2.9.x script does.
# Define User Variables
:global ddnsuser "xxx"
:global ddnspass "xxx"
:global ddnshost "xxx.xxx.com"
:global ddnsinterface "gw-inet.mojiro.awmn"
# Define Global Variables
:global ddnsip
:global ddnslastip
:if ([ :typeof $ddnslastip ] = nil ) do={ :global ddnslastip "0" }
:global ddnsinterface
:global ddnssystem ("mt-" . [/system package get system version] )
# Grab the current IP address on that interface.
:global ddnsip [ /ip address get [/ip address find interface=$ddnsinterface ] address ]
# Did we get an IP address to compare?
:if ([ :typeof $ddnsip ] = nil ) do={
:log info ("DDNS: No ip address present on " . $ddnsinterface . ", please check.")
} else={
:if ($ddnsip != $ddnslastip) do={
:log info "DDNS: Sending UPDATE!"
:log info [ :put [/tool dns-update name=$ddnshost address=[:pick $ddnsip 0 [:find $ddnsip "/"] ] key-name=$ddnsuser key=$ddnspass ] ]
:global ddnslastip $ddnsip
} else={
:log info "DDNS: No update required."
}
}