Custom DOI resolver (tested for Springerlink and ScienceDirect), compatible with TUM eaccess proxy.

AlternativeDoiResolver.js

// ==UserScript==
// @name     AlternativeDoiResolver
// @version  1
// @grant    GM.xmlHttpRequest
// @description Converts the pretty useless DOI links to clickable links ot a custom resolver.
// @include *sciencedirect*
// @include *springer*
// @include *ieeexplore*
// @include *acm.org*
// @require https://code.jquery.com/jquery-3.5.1.min.js
// ==/UserScript==

jQuery('head').append('<style type="text/css">a.doilink { background-color: rgba(245, 132, 66, 0.5); border-radius: 0.5em; padding: 0.3em; margin: 0.8em; font-weight: bold; display: block; }</style>');


function getUrl() {
    GM.xmlHttpRequest({
        method: "GET",
        url: "https://sci-hub.now.sh",
        onload: function(response) {
            console.debug("Got response from sci-hub dispatcher.");
            var urls = $(response.responseText).find(".biglink").map((index, elem) => new URL($(elem).attr('href')));
           	do_resolve(urls);
        },
        onerror: function(errsponse) {
            console.error("Failed to access sci-hub dispatcher, error ${errsponse.statusText}. Using default value.");
            do_resolve([new URL("http://sci-hub.tw/")]);
        }
    })
}

function do_resolve(resolver_urls) {
    
  function updateLink() {
        // Using XPath
        // Allows us to get span or a results.
        var result = document.evaluate('//*[(self::span or self::a) and (starts-with(text(),"https://doi") or starts-with(@href, "https://doi"))]', 
                                       document, 
                                       null,
                                       XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, 
                                       null);
        console.debug("Found " + result.snapshotLength + " results.");
        if (result.snapshotLength == 0) {
            console.warn("No links found, retrying in a second.");
            setTimeout(updateLink, 1000);
            return;
        }
        for (var i = 0; i < result.snapshotLength; i++) {
            var node = result.snapshotItem(i);

            var newlink = document.createElement("a");
            var url;
            if (node.tagName == "A") {
                // Take href of node
                url = new URL(node.href);
            } else if (node.tagName == "SPAN") {
                url = new URL(node.innerHTML);
            } else {
                console.error("Unknown element type " + node.tagName);
                return;
            };
          	console.log(resolver_urls);
          	var fragment = jQuery('<p></p>').insertAfter(node);
          	$(resolver_urls).each((index, resolver_url) => {
              	console.log(resolver_url);
              	var final_url = new URL(resolver_url);
              	if (final_url.pathname.endsWith('/')) {
                  console.log("Final url pathname: " + final_url.pathname);
                  final_url.pathname = final_url.pathname.slice(0, -1) + url.pathname;
                } else {
                  final_url.pathname = final_url.pathname + url.pathname;
                }
              	jQuery('<a href="' + final_url.href + '" class="doilink">' + resolver_url + '</a>').appendTo(fragment);
            });
          	fragment.hide().insertAfter(node).fadeIn(1000);
        };
    }

    console.log("Setting update timeout.");
    setTimeout(updateLink, 1000);
}

jQuery(getUrl);

Added ieeexplore functionality.

Added animation.