Update PostGIS GEOS library on RDS

Hi,

Are you sure you're running RDS Postgres and not Aurora Postgres? Based on the postgis_full_version I can't tell which engine or version you're using but RDS Postgres 13.5 has the following GEOS version available.

postgres=> select version(),postgis_full_version();
                                                 version                                                 |                                        postgis_full_version
---------------------------------------------------------------------------------------------------------+---------------------------------------------
 PostgreSQL 13.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11), 64-bit | POSTGIS="3.1.4 ded6c34" [EXTENSION] PGSQL="130" GEOS="3.9.1-CAPI-1.14.2" PROJ="Rel. 5.2.0, September 15th, 2018" LIBXML="2.9.1" LIBJSON="0.13.1" LIBPROTOBUF="1.3.2" WAGYU="0.5.0 (Internal)"
(1 row)

Aurora Postgres 13.4 has the following GEOS version.

postgres=> select aurora_version(), version(), postgis_full_version();
 aurora_version |                                             version                                             |                                                                          p
ostgis_full_version
----------------+-------------------------------------------------------------------------------------------------+-----------------------------------
 13.4.0         | PostgreSQL 13.4 on x86_64-pc-linux-gnu, compiled by x86_64-pc-linux-gnu-gcc (GCC) 7.4.0, 64-bit | POSTGIS="3.1.4 ded6c34" [EXTENSION] PGSQL="130" GEOS="3.9.1dev-CAPI-1.14.1
" PROJ="6.2.1" LIBXML="2.9.9" LIBJSON="0.12.99" LIBPROTOBUF="1.3.0" WAGYU="0.5.0 (Internal)"
(1 row)

I'd recommend testing your postgis query on a test copy of one of these versions and if appropriate you could perform an engine version upgrade to pickup the required GEOS/postgis version.