how to limit a parameter within an adjustment code

Question

I need to constrain the parameters a and m within the following least squares fitting code of a nonlinear curve, I am using R

set.seed(1)

pacman::p_load(gslnls)

n <- 50 

Datos <- data.frame(
            theta = (1:n) / n,
            Xb_exp = 2.5 * exp(-1.5 * (1:n) / n) + rnorm(n, sd = 0.1))

plot(Datos$theta,Datos$Xb_exp)

Ajuste<- gsl_nls(fn=y~1-exp(-a*((x-(-2))/length(Xb_exp))^(m+1)),
data=data.frame(y=Datos$Xb_exp, x=Datos$theta), start=c( a= 1, m= 1), algorithm = "lmaccel",  control = gsl_nls_control(scale = "levenberg",  avmax = 0.75),trace = TRUE,)

summary(Ajuste)

a<- predict(Ajuste, interval="prediction", level=0.99)

a<- a[, -c(2,3)]

Datos<- dplyr::mutate(Datos, Xb_mod=a)
plot(Datos$theta, Datos$Xb_mod)