Hello gyus, I've been a Mikrotik fan for some time but now I'd really appreciate your help as I wasn't able to find any guide on this one:

I have a wireless ISP (they run the whole network on Mikrotiks too) which gave me 2 IP addresses, each one is tied to one account, each with different speed. But there is only one cable going from the roof. Different devices on my network are supposed to use different paths.

CURRENT SETUP:

Until now, I plugged in the ISP cable to Mikrotik (RB2011L) and bridged 2 ports, using the second port in the bridge to connect a second router (here let's call it "RouterX").

First IP address from my ISP: 10.51.99.101
Second IP address from my ISP: 10.51.99.102

ISP Gateway: 10.51.99.1, Netmask 255.255.255.0 - same for both connections

Mikrotik port 1: IP 10.51.99.101
Mikrotik port 2: no ip, bridged with Mikrotik port 1, cable from this port is plugged into Router X.
Router X WAN IP: 10.51.99.102

- Devices connected to Mikrotik get internet access 10/5 Mbps because they go out through Mikrotik (10.51.99.101)
- Devices connected to RouterX get internet access 15/10 Mbps because they go out through RouterX (10.51.99.102) but end up in the same cable to the roof where the ISP's antenna is

WANTED SETUP:

What I'd like to achieve is get rid of RouterX and use only 1 Mikrotik port for the ISP connections. I thought I'd assign both ISP IPs to the same interface but then I don't know how to tell which devices will use which connection.

Here is what I need:

Mikrotik port 1: one cable from ISP will connect here and it will have both IPs assigned
Some devices will use the 10/5 Mbps connection while some will use the 15/10 Mbps connection

But how do I determine which device should use the given connection (10.51.99.101 or 10.51.99.102)? The ISP needs to see somehow whether the connection is coming from 10.51.99.101 or 10.51.99.102, but I really don't have a clue how to do this, using only 1 Mikrotik port.

Thank you!

as i understand you are using NAT (masquerade) and you have your local network. If so, you can set src-nat rules instead and separate - what addresses are NAT'ed to what IP address. IMO nothing else is required. Since this is one cable and you can assign both IP addresses on same interface without any problems.