Mikrotik as a gateway in front of the gateway (gee) - how?

Question

Mikrotik

Greetings.
I have - hap ac lite, its address .7.99
ether1-gw is connected to the adsl modem, whose address is .7.1 The
config is not very complicated - briefly several ppp l2tp tunnels are raised, then using mangle mark + routing they are tied to physical ports (eth3 <- > ppp1 etc.).
Task:
wlan1 is free. It is necessary that by connecting to wlan1 AND having dhcp enabled, the client simply gets into the modem's lan and receives all the settings from it. (it's trivial to bridge wlan1 and eth1-gw).
BUT if the client connects with a manually configured IP, dns AND (!) gateway is aimed at the Mikrotik address (.7.99), then Mikrotik should become a full-fledged gateway for him. The bottom line is to get the ability to mangle packets from such a client, which will make it possible to divert packets by ip list to no one on the ppp interface. The rest we skip to .7.1
How would I do it somehow - weakened the rules of drop and so on. But it works very slowly, speedtest.net could not wait (although there is an Internet in general and even packets to addresses from the list fly as they should through ppp). Where is the loop? Plug?
Can you help? Thanks

Answer 1

https://www.reddit.com/r/mikrotik/comments/30l0z6/...

Answer 2

BUT if the client connects with a manually configured IP, dns AND (!) gateway is aimed at the Mikrotik address (.7.99), then Mikrotik should become a full-fledged gateway for him.

Do you understand that you get two networks in one broad domain? So to put it mildly, it is not necessary. Please draw a diagram and in more detail what you want in the end.
Most likely, everything is done much easier and without any markings.
For example, through vlan or by connecting to another port. As an option, you can create two wifi networks that will resolve to different networks.