Photos have diseappeared from my iPhone and iPad.

kaye_martins wrote: …My iCloud might have been full during that period but that doesn't explain why the photos would simply vanish.

Well, if your iCloud storage is full, and you've told your phone to transfer stuff to iCloud, then that sure could account for your pictures vanishing. Having less than 10% free space on either your phone or at iCloud.com can inhibit the phone's ability to process file. How much free space do you have on the phone?

Unfortunately, if you don't keep your own backups to an external drive, then there really isn't any place else to look. To back up your pictures you can just plug in an external drive, select the pictures, and use Share>Save to Files. Alternatively, you can backup to a computer:

Transfer photos and videos from your iPhone or iPad to your Mac or PC

If you haven't done one of those backups, then your remaining pictures continue to be in peril. 

kaye_martins wrote:

I'm using 247.2GB out of the 256GB that I have on the phone and iCloud is 154.4GB out of 200GB...

More storage at iCloud doesn't add any to the iPhone. With 256 GB, you should keep under 225 GB used.

t I don't understand why the phone would remove photos from specifically those months

When some app demands storage, the iPhone moves files around in an attempt to accommodate. If there's not enough room, files can be lost. I can see how this might affect a connected group of files, but we can't really know. I've never had pictures disappear leaving a mystery-- for me it's been drive failures or personal fumbling. We've all had losses from our computers, and I'm afraid that the only insurance is having good backups. I know that pictures are especially precious…