If -1 is really the PHP setting, then this should mean that PHP can use "unlimited" memory.
The limitation must then be the free memory. If it cannot allocate more than 700MB of RAM, it must be all that was left available.
If you have dedicated RAM, you could try shutting down services or other features to free up RAM.
If the RAM is shared with other websites, you might have to wait for a time where less RAM is used by the other customers.
If those are not possible, you might need to find a hosting with more/dedicated RAM.
On shared hostings, swap is usually not an option. It's either included, or it's not. Having little RAM but access to swap can work, but might be slow.
