Horizon View 5.x went end of general support on 2016/09/14 and 5.1.3 was technically not supported long before that (support applies to the latest maintenance release). Horizon View 6.x is going end of support on 2019/06/19. You need to prioritize an upgrade to the latest Horizon 7.x release. Given your situation I would advise a greenfield deployment and migration. Server 2019 support for RDS was only just added in 7.7.0.

https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/support/product-lifecycle-matrix.p...

Release Notes for VMware Horizon 7 version 7.7