FormHooks.php

<?php

namespace Myself\MySitepackage\Hooks;

use TYPO3\CMS\Form\Domain\Model\Renderable\RootRenderableInterface;
use TYPO3\CMS\Form\Domain\Runtime\FormRuntime;

/**
 * @author Martin Terber <martin.terber@ppw.de>
 */
class FormHooks
{
    /**
     * @param \TYPO3\CMS\Form\Domain\Runtime\FormRuntime $formRuntime
     * @param \TYPO3\CMS\Form\Domain\Model\Renderable\RootRenderableInterface $renderable
     * @return void
     */
    public function beforeRendering(FormRuntime $formRuntime, RootRenderableInterface $renderable)
    {
        if ($renderable instanceof \TYPO3\CMS\Form\Domain\Model\FormDefinition) {
            if (count($renderable->getPages()) > 1 ) {
                $pages = $renderable->getPages();
                $renderable->setRenderingOption("navigation", $pages);
            }
        }
    }

}

Use it in your Form.html fluid template like that:
(navigation links to be added)

<ul class="form-navigation">
    <f:for each="{form.renderingOptions.navigation}" as="navItem" iteration="i">
        <li>
            <f:if condition="{navItem.index} < {form.currentPage.index}">
                <f:then>
                    <button formnovalidate="formnovalidate" type="submit"
                            name="tx_form_formframework[myformname-123][__currentPage]"
                            value="{navItem.index}">{i.cycle}. {navItem.label}
                    </button>
                </f:then>
                <f:else>
                    <span>{i.cycle}. {navItem.label}</span>
                </f:else>
            </f:if>
        </li>
    </f:for>
</ul>

In your ext_localconf.php:

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['ext/form']['beforeRendering'][1613033383] 
    = \Myself\MySitepackage\Hooks\FormHooks::class;