I would check in at the business counter. This worked for me with Brussels Airlines flying economy on first leg and Polaris from Brussels to Newark. Perhaps LH is more strict. I did have to ask for a “priority” tag on my checked luggage (United Premier Silver), which was denied, however it was on my luggage when it appeared in Newark.
The funny thing regarding luggage allowance was that the United website said two different things, 1 free on my ticket receipt and 2 free when using the checked luggage tool. Since I only had one, this was not an issue.