Beginner Java Recursive Linked Lists Problem

I'm trying to grasp the concept of Recursion and do as much practice as I can but I seem to be completely stumped on it when it comes to more difficult issues such as Recursive functions and using it in Linked Lists and Trees.

Here I am simply trying to reverse a linked list, using recursion at the leetcode question available here - https://leetcode.com/problems/merge-two-sorted-lists/. While there are solutions available, I'd like to know if possible as to why my code doesn't work as when I print out the values the correct ones seem to show up but I can't figure it out why it is not linking properly. Any help is appreciated.

Thank you

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution 
{
    static ListNode temp = new ListNode();
    
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) 
    {
        //Base case, return temp listNode
        if(l1 == null && l2 == null) return temp;
        
        //If l1 has been used up and l2 not
        if(l1 == null && l2 != null)
        {
            temp.next = l2;
            l2 = l2.next;
        }
            
        //If l2 has been used up and l1 not
        if(l2 == null && l1 != null)
        {
            temp.next = l1;
            l1 = l1.next;
        }
        
        //If both nodes exist
        if(l2 != null && l1 != null)
        {
            if(l2.val <= l1.val)
            {
                temp.next = l2;
                l2 = l2.next;
            }
            else 
            {
                temp.next = l1;
                l1 = l1.next;
            }
        }
        
        System.out.println(temp.val + " next " + temp.next.val);
        
        temp = temp.next;
        
        return mergeTwoLists(l1,l2);
    }
}