Thanks for the pointer, John.
Iíve turned off all modules within the responsivizer template.
It looks like an issue with the existing site template.
This is the error message I see in the Joomla error log:
[12-Feb-2019 05:55:06 UTC] PHP Fatal error: Call to undefined function wrap_with_tag() in /home/lovefinder/public_html/templates/theme1568/html/com_content/category/category.php on line 23
When I query that file I see this code:
<?php echo wrap_with_tag(wrap_with_span($this->escape($this->params->get('page_heading'))), $template->params->get('categoryPageHeading')); ?>
Thatís where the wrap_with_tag function is called.
Iíve deleted that one line of code to see what the impact would be to the site and there appears to be no impact. Once that line of code was deleted, the device emulator and phone browser started working.
Iíll make a note of the changes in case I need them in the future.
Thanks again for your assistance. Seemed to have found a way forward.