Equal internet allocation per user

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

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.

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

rfc3942 allows for assigning an integer as an option