I think, if you have just started Cimzia, you should give it time before making any changes. When your doctor prescribed it, did he/she give you an estimated timeframe for it to show results? My wild guess (I'm not a doctor, and no training whatsoever, and no experience with Cimzia) is that 3 month (90 day) trial period is probably standard. You might want to check with the Cimzia threads to see if others with experience can give you some idea of how long it took to work for them. Bear in mind, everyone is different, and individual cases cover the spectrum in severity, intensity, complications...
I'm no expert on anything... and my knowledge of how LDN works is partly anecdotal, with some parts being little more than folklore, and some taken from posts, articles, etc., from doctors, scientists, or just other sufferers of this disease. There are very few 'real' experts in LDN, and the interaction between it and other medications. My own personal take is that... the healthier, more robust ones immune system is, the better that LDN seems to work. When I first started on it, if I got flu, or bad cold, anything that would put a strain on my immune system, I would experience some minor flaring. However, as soon as the cold/flu whatever passed, my flare would go away very fast. As years passed, these became fewer and fewer... until now, if I catch a bug, it doesn't affect my Crohns at all. The reason I raise this.. and type so much extra reading..is that I don't know what the effect of large doses of Pred or Cimzia might have on your immune system. It probably won't affect the LDN... but if the LDN works better with a strong immune system... well, you can see there is a potential for troubles in that case.
If neither high doses of Pred or Cimzia suppresses the immune system, it shouldn't make a difference. I just don't have the knowledge or expertise to make that assumption. And if a doctor is mistaken, (about the mechanics of how LDN works its magic) then this may not be your best choice. Thing is... a doctor MAY look at the drug desk reference and there would be no red flags raised by taking both... without stopping to consider if what one drug does to the body might offset the potential benefits of the other. That is the big conundrum. I'm not saying it would, I'm saying I don't know, and I'm not entirely sure the 'experts' do either.