Difficulty of getting annex 49 in time and start date of the contract

Hello,

I'm in a bit of a difficult situation because of the administration and I'd greatly appreciate some answers or clarifications.

I've got a 2 year postdoc at VUB and I'm planning to arrive in Belgium on July 15. I got the D visa, hosting agreement, annex 46 (not annex 49) and contract. The HR, whom I find extremely unresponsive generally, told me that I could start the contract on July 15. But I'd appreciate it if you could please answer the following very important question, that hasn't been responded to yet by HR:

Can I start working on July 15, without registering at the municipality on the same day, i.e. without having an annex 49? Please note here that I've already obtained a D visa, positive decisions on work and residence permits (annex 46), and hosting agreement. The HR can obtain a national number (SSIN) through  https://www.belgianidpro.be/en/page3.html , and for this my presence in Belgium isn't required to get a temporary SSIN (please correct me if I'm wrong). So it appears that the HR should be okay with me starting on July 15 without any additional documents.

The reason behind this important question is that I need a long term address for the registration at the municipality, and for this, I'd need an apartment in Brussels in the first place, and the latter is nearly impossible because:

i) I'm in Germany, and finding an apartment in Belgium without being present there carries a risk of getting scammed in the process, and

ii) chance of me finding an accommodation without having a signed contract is fairly slim to none. So unless my contract starts and I'm on VUB's payroll, I can't rent an apartment, nor can I open a bank account without an SSIN. And this means I can't register at the municipal office. Am I wrong?

It seems to me that in order to solve problem A, I need to solve problem B and vice versa, so which one do I solve first?

Thank you sincerely for any helpful information!