Dear community I would like to know the technical opinion on the following problem

Current Scenario
Tplink Ac1200 -Gateway Lan router (16 users)
150Mbps Fiber Internet - In use
20Mbps Dsl (Not used)

Required Scenario (already configured)
TP Link TL 470t Balancer (New Lan Gateway)
150Mbps Fiber Internet + Balanced 20Mbps Dsl Service (configured failover) Wan1 Fiber 150 Wan2 Dsl 20

Problems
-When the Wan1 is connected directly to the Balancer and the speed is measured, the maximum is 35 - 40 Mbps of 150 contracted
- It was tested Connecting the Wan1 From the Tplink Ac1200 and not directly to the balancer, and the result is the same 35-40 Mbps of 150 Contracted
The Wan and Lan ports of the Balancer all work at a maximum 100Mbps, it is understood that logically it will never navigate at 150, but shouldn't it deliver a maximum of 100? Does anyone know that it should be adjusted so that the 60% of contracted bandwidth is not lost?
When you work only with Wan 2 (Dsl) there is no problem in fact with the 2 Wan either only that it is not possible that only with Wan 1 attached to the balancer it navigates at more than 50Mbps