When working with AT#XHTTPCREQ we have encountered the following provlems:
1. Sending AT Command `AT#XHTTPCREQ="POST","/upload","Content-Length:3357",3357` with the payload of size 3357 in one line, it breaks the serial_lte_modem app. This limit is not mentioned in the docs https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/applications/serial_lte_modem/doc/HTTPC_AT_commands.html I think it is caused by the handling of buffer overflow - it only drops the last character, but the app itself never recovers from it.
2. If we send the payload chunked, we encounter problems with unpaired double quotes - again, the app is deadlocked.
You can reproduce it by sending this: `{"data":"3582e2192900` as payload.
Caused by this line: https://github.com/nrfconnect/sdk-nrf/blob/master/applications/serial_lte_modem/src/slm_at_host.c#L565