I can't fragment my packet on a TCP connection

Question

I've been trying and trying to fragment my TCP packets but I havent found any helpful implementation of it. I am familiar with the theory and concepts of fragmentation, have even come across some flags such as IP_PMTUDISC_DONT, IP_PMTUDISC_WANT, and IP_PMTUDISC_DO and set them but the wireshark's capture always showed DF Flag as on.

I've set the MTU of my 'lo' Network Interface to 1500 since I'm using LoopBack Address on both, the server and the client. And I thought that fragmentation will be handled by the Network Layer, but thats not the case I guess... Please help me with fragmentation of the packet.
Here's my code...

Server.cpp

#include <sys/types.h>
#include <sys/socket.h>
#include <iostream>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netdb.h>

int main(){

    struct sockaddr_in serv_addr, cli_addr;
    char buff[255];

    int FileDesc = socket(AF_INET,SOCK_STREAM, 0);
    if(FileDesc < 0){
        perror("Socket Creation Failed. ");
        exit(EXIT_FAILURE);
    }

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(5455);

    if (bind(FileDesc, (const sockaddr*)&serv_addr, sizeof(serv_addr)) < 0){
        perror("Bind Failed.");
        exit(EXIT_FAILURE);
    }

    if (listen(FileDesc, 6) < 0){
        perror("Listen Failed.");
        exit(EXIT_FAILURE);
    }
    
    socklen_t cliLen;
    int AcceptFD = accept(FileDesc, (sockaddr*)&cli_addr, &cliLen);
    if(AcceptFD < 0){
        perror("Accept Failed.");
        exit(EXIT_FAILURE);
    }
 
    //Setting the MTU
    struct ifreq ifr; 
    ifr.ifr_addr.sa_family = AF_INET;//address family
    strncpy(ifr.ifr_name, "lo", sizeof(ifr.ifr_name));//interface name where you want to set the MTU
    ifr.ifr_mtu = 1500; //your MTU size here
    if (ioctl(FileDesc, SIOCGIFMTU, (caddr_t)&ifr) < 0){
        perror("ioctl error.");
        exit(1);
    }
    std::cout << ifr.ifr_ifru.ifru_mtu;
    
    while(1){
      //File Transfer
    }

    close(AcceptFD);
    close(FileDesc);

    return 0;

}

Client.cpp

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <iostream>
#include <unistd.h>
#include <netdb.h>
#include <string.h>
#include <arpa/inet.h>

int main(){

    struct sockaddr_in serv_addr;
    char buffer[255];

    int fd = socket(AF_INET, SOCK_STREAM, 0);
    if (fd < 0){
        perror("Socket Creation Failed.");
        exit(EXIT_FAILURE);
    }

    FILE *fp;
    char *filename = "File.txt";

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    inet_aton("127.0.0.1", &serv_addr.sin_addr);
    serv_addr.sin_port = htons(5455);

    int conn = connect(fd, (const sockaddr*)&serv_addr, sizeof(serv_addr));
    if (conn < 0){
        perror("Connect Failed. ");
        exit(EXIT_FAILURE);
    }

    while(1){
       //File Recieve
    }

    close(fd);
    return 0;

}

This also begs the question, how would I re-assemble my packet back in the same order? Thank youu.