Using 2 wireless APs connecting each other with Ethernet to have a 4 bands device

Hi, since I can't have a wired backhaul can I take a spare AP I have, make it a dumb AP with mesh but without DHCP, firewall and use its port to connect another AP which will serve the same wifi but on a different channel?

Yes, but why not share it from the one used as uplink ?

In my case having a Flint 2 and a Cudy WR3000H I would have to use the same channel for the backhaul and AP since they only have 2 radio each. What I've understood is that I would split the bandwidth. Unfortunately I need 2.4ghz as well as 5ghz to be extended.
I'm still wondering whether I should return a cudy but Ive paid it only 40€

Yes the uplink radio speed is split between the upstream client and AP.

Haven't checked the specs for the radios, but the Cudy is half the speed of the Flint 2, meaning in a best case scenario, it would perform similar to the Flint 2 if used as described earlier.

The flint is my main router but the idea would be to use my old fritz devices with the cudy or the TP-Link re650 but that one should have 4 radios

You should focus on getting coverage where needed, not maxing out 24x7 electronics number.

yeah that's exactly the point, if to install an AP i need to halves its bandwidth it means that i need to put it where the signal is at least 40Mbps which is not an ideal place for the range i have to cover. Instead i can afford to move it closer if they are on different channels