Equal internet allocation per user

Installing and Using OpenWrt Network and Wireless Configuration
1

Good afternoon I am interested in information on how to assign fixed bandwidth for each user that connects, for example 128k of upload and 300k of download

2

You can use QoS for that task. But instead of using the SQM (which will ensure fairness, not fixed quota), you should use nftables/netfilter.
Available package is called nft-qos.

3

Good idea, but I would like to have assigned a certain amount of kbits to each user connected by dhcp

4

rfc3942 allows for assigning an integer as an option