The router, like a signal amplifier, blunts and slows down, what's wrong?

Question

DHCP

Hello! Should I reserve a DHCP address for the router (which is already a repeater) in the main router? Will it help for more stable operation? The Internet comes from neighbors, we have TL-WR841N, they have TL-WR740N, everything seems to be installed as it should! I put it as a "signal amplifier", then everything is according to the instructions! but the Internet hangs for a while, then it slows down terribly! I changed my Smart IP to Static IP, and the Internet began to “fly”, after half a day, however, it began to “load” again. So I think maybe the neighbors need to register our router and the picture will be better? And I also noticed that in their DHCP list my phone and iPad have two mas addresses and ip, is this normal? And my router is also among the devices, sometimes it disappears, although in fact it works! My router is also on the client list! I'm already tired of rebooting our router every day,