Multiple Gateways (PPPOE) issue

Hi Everyone, I have some questions with setting up Multiple gateways (one of the gateways is PPPOE).

Current:
ISP modem (PPPOE) ↔ Mikrotik RB2011 ↔ Computers (LAN)

Now I am planning to add a new testing router under RB2011. For some reasons, I do not want to add an extra switch between my new router and computers. I want to do something like this:

There are total 10 ports on RB2011 and 2 ports (LAN + WAN) on my new router:
Port 1 - 7: LAN (each has separate subnets, from 192.168.21.0/24 to 192.168.27.0/24 respectively)
Port 8: Lan of new router (192.168.28.0/24, 192.168.28.2 is the LAN ip of my new router)
Port 9: Wan of new router (10.0.0.0/24, NAT on my new router)
Port 10: ISP modem (PPPOE)

What I want to do:
When my Port 1 192.168.21.0/24 want to access internet, I want this to go through my new router first (which is port 8 192.168.28.2), then come out from WAN of my new router and come back to my RB2011 port 9, and finally go to Port 10 PPPOE for internet.

When my Port 2 192.168.22.0/24 want to access internet, it go out directly to Port 10 PPPOE gateways for internet.

I have done some research but seems I cannot set multiple gateways as one of the gateways is PPPOE which has dynamic ip address.

I am not sure if this is possible, is there any suggestions? Thank you.