Apart from being right under the street sign you may have big problems with the word Plaque. Generally considered to be attached to a wall or surface. Rather than affixed like a sign
you may have a better outcome with using “Historical Sign”
I have seen similar signs get in as a Waypoint using that naming convention
Title
Biesenger Historical Sign.
This sign provides information about the origin of the name Biesingerstraße. It is named after the “Im Biesinger” estate, which comes from the Biesinger family, who originally owned this land. The land was was developed in the late 19th century and called Biesingerstraße.
In supporting.
Great place to explore. These historical signs provide a path to explore through the area. Evidence for X can be seen in street view (provide link to street view view). Evidence of the signs can be foun d here (insert link) and a breakdown on the history associated with the sign and area is here (insert link) and here (insert link).
Don’t quote me. 
I am only suggesting ideas. They may not work. But try a different tack.
This is my go to process for creating nominations. It is a cut and paste. You probably already know it all so my apologies if teaching you to suk eggs
Write brief title, description, supporting (make sure this text is not rubbish but at least relevent). Select what it is (cafe etc) - with photo. /you can take photos before hand. In particular for main photo. Edit so they fit best. 4x4 (square is best) Centre what the POI is. You can upload from your gallery when creating the nomination. This main photo size will suite the system better.
For this sign. Doing everything to make the main photo pop is good. Because it is a dirty sign AND sits under a steet sign. So use what nature gives you. Blue sky, sun set, wait till leaves out. Make that contrast work.
Now you have created the nomination. DO NOT Upload now. Use the Upload Later. Get home. Log into WayFarer on your PC/Laptop. Go to the Contributions Section. Now on phone click upload now. Refresh the contributions every now and then and after a while your nomination will appear. Put it on Hold. Once in hold you can edit at will.. ((( note every now and then Niantic will grab your nomination before you edit but hey that happens - not often - I just roll my eyes - take it and nominate again 
however it is why I still write basic good text in the first instance)))
Once on hold. This means you can rewrite the title, description and supporting information. You can do your research. Include weblinks etc to your nominations (if they exist). Remember in supporting. Select one of the criteria. Say It is a great place to explore. Because of X Y Z. Or Great Place to Socialise. Evidence to support this goes a long way. DO NOT say make a great pokestop. Or we need more stops etc. This is not good evidence or supporting information.
Before releasing hold. If it is more challenging nomination. I come to the nomination support forum. Include all text, images and include the address (try to include the actual text (allows for easier translation) and direct link to Gmaps). And then ask hey how can I improve my nomination. I have often used this forum to great impact. Steadily increasing the quality and subsequent success of my nominations.
Once you have your feedback and you ready click - release hold.
May not work. But what we have done is spent some time to polish our nomination up to give it - its best chance.
Oh and read the criteria . Often (all four sections)
And really read these Criteria Clarification Collection - Niantic Wayfarer Community
There are 18 sections there - that could provide insight and help when thinking of what to nominate and how to write that nomination
Pay just as much attention on what does not qualify.
And this is a great article on how to write a description. Wayspot Title and Description Tips – Every bit helps particularly with nominations that reviewers get challanged by
Remember - and sadly - some reviewers are rushing the system. They want their in game rewards. Quality or qualification mean less. At scale this means more rubbish in systems or more rejections of valid qualifications that need effort to review. That means as a nominator we work just a little bit harder to ensure we give our nominations its best chance.
Good luck @wooperIooper