If you’re running a mail server at your site, then the ISP needs to allow port 25 for you, or you need a smarthost operating outside of the ISP’s network which listens on some other port that your local server is configured to use as well. There’s no way for your server to use a non-standard port to deliver mail to the world in general, because EVERYONE would need to be listening on that unstandard port.
When an ISP blocks port 25, normally they have a mail relay on their network that you are allowed to use as a smarthost.
Ask them the IP or name of that relay and configure it on your system.
When you really want to solve it in the router, you could of course setup a dstnat to map all outgoing port 25 connects to that IP address.
However, I would not advise to do that as it could result in difficult to debug problems and/or complaints from users/customers.
Another ISP(receive/send email, correct dns record). Create a tunnel between new isp and main isp.
From new isp->routeros->ip tunnel->old isp(if not block ipsec/gre)->routeros->mail server